The word used to denote in-laws in Arabic is Sihr. It is mentioned in al Qamus al Muhit:
Al Sihr (with a Kasra): Relative and bond through marriage. Its plural is Ashar and Sahra’. It also refers to one’s son-in-law and brother-in-law (sister’s husband)…
In Mujam al Wasit it states:
Ashara ilayhi means he drew closer to him, and to his people; i.e. he married into their family.
Lisan al ‘Arab makes a slight differentiation between the words Ashar and Akhtan:
Ashar are the family of the wife and the family of the husband are only referred to as Akhtan.
He then presented the commentary of al Fara’, al Zujaj, al Azhari, and others on the verse:
وَهُوَ الَّذِي خَلَقَ مِنَ الْمَاءِ بَشَرًا فَجَعَلَهُ نَسَبًا وَصِهْرًا
And it is He who has created from water a human being and made him [a relative by] lineage and marriage.
Ibn Hajar al ‘Asqalani explains the meaning of the word sihr in the verse above:
Sihr refers to both the family of the wife and husband. There are some who limit its implication to the family of the wife… The true meaning of Musaharah is closeness. Al Raghib said, “Al Sihr are those related to the husband through marriage (his son-in-law, brother-in-law, etc.)… Ibn Al A’rabi said, “Ashar are those prohibited through closeness or ancestry or marriage…”
The Prophet salla Llahu ‘alayhi wa sallam established marital relationships with his Sahabah radiya Llahu ‘anhum, which is why Safi al Rahman al Mubarakpuri says:
The Messenger salla Llahu ‘alayhi wa sallam established a familial bond with Abu Bakr and ‘Umar by marrying (their daughters) Aisha and Hafsah; and similarly, marrying his daughter Fatimah radiya Llahu ‘anha to ‘Ali ibn Abi Talib radiya Llahu ‘anhu, and his two daughters, Ruqayyah and Umm Kulthum, to ‘Uthman ibn ‘Affan gives an indication that he desired to strengthen ties with all four men, whose suffering and sacrifices for Islam in the crises they endured is well known… The practice of the Arabs was to honour one’s relations through marriage, and such marital relationships was seen by them as an avenue through which various tribes may draw closer. They regarded opposition and fighting with one’s in-laws as a shame and disgrace…
This indicates to us another reason for the Prophet’s salla Llahu ‘alayhi wa sallam marriages to the Mothers of the Believers.
Amongst the Prophetic traditions which shed the most light in this regard is what has been narrated by ‘Abdullah ibn Abu Awfa from the Prophet salla Llahu ‘alayhi wa sallam:
I asked my Rabb that whoever I marry from my Ummah or get him married to be with me in Jannat, and He granted it.
Whoever will ponder over the verse:
وَالطَّيِّبَاتُ لِلطَّيِّبِيْنَ وَالطَّيِّبُوْنَ لِلطَّيِّبَاتِ أُولٰئِكَ مُبَرَّءُوْنَ مِمَّا يَقُوْلُوْنَ
And pure women are for pure men, and pure men are for pure women. These are declared innocent of what the slanderers say.
will know full well that the blessed and pure Prophet of Allah salla Llahu ‘alayhi wa sallam would not marry accept one who is pure, and only those who are pure will be allowed to share in his lineage. Allah subhanahu wa ta ‘ala would never disgrace his blessed Prophet salla Llahu ‘alayhi wa sallam in this regard.
Hereunder follows some of the marital relationships between the Prophet salla Llahu ‘alayhi wa sallam and the Sahabah radiya Llahu ‘anhum.
The pure and blessed Prophet salla Llahu ‘alayhi wa sallam married the pure daughter of Sayyidina Abu Bakr radiya Llahu ‘anhu, Sayyidah Aisha al Siddiqah radiya Llahu ‘anha, a fact that is not denied by any of the scholars, whether they be pioneers in biographies, history, or genealogy. Nevertheless, we will quote here those textual evidences which establish that this union was upon the instruction of Allah subhanahu wa ta ‘ala and from His Infinite Wisdom.
Muslim has reported in his Sahih”
Aisha radiya Llahu ‘anha reported that Allah’s Messenger salla Llahu ‘alayhi wa sallam said to her, “I saw you in a dream for three nights when an angel brought you to me in a silk cloth and he said, ‘Here is your wife,’ and when I removed (the cloth) from your face, lo, it was yourself, so I said, ‘If this is from Allah, let Him carry it out.’”
The mother of Sayyidah Aisha al Siddiqah radiya Llahu ‘anha is Umm Ruman, the daughter of ‘Amir, the son of ‘Uwaymir, the son of ‘Abdul Shams, the son of ‘Itab, the son of Adhinah, the son of Sabi’, the son of Dahman, the son of Harith, the son of Ghanam, the son of Malik, the son of Kinanah. Her ancestry meets with that of the Prophet salla Llahu ‘alayhi wa sallam at Kinannah. She is a Sahabiyyah radiya Llahu ‘anha.
He married Hafsah bint ‘Abdul Rahman ibn Abi Bakr al Siddiq, the granddaughter of Sayyidina Abi Bakr radiya Llahu ‘anhu.
Ibn Habib mentioned that she was married to Sayyidina Hussain radiya Llahu ‘anhu, as opposed to Sayyidina Hassan radiya Llahu ‘anhu; however, I assume this to be an error.
He married Umm Hakim bint Qasim ibn Muhammad ibn Abi Bakr al Siddiq, who is the sister of Umm Farwah who will be discussed shortly.
However, Muhammad A’lami al Ha’iri wrote his name as Ishaq ibn Jafar ibn Abi Talib. This is an obvious error as the Ishaq above was actually the son of ‘Abdullah ibn Jafar ibn Abi Talib, as attested to by all the books of genealogy. This Ishaq was titled al ‘Uraydi.
Ibn ‘Inabah records under the children of ‘Abdullah ibn Jafar ibn Abi Talib:
From them (his children): Ishaq al ‘Uraydi, whose mother was an Umm Walad.
Also attesting to this is:
Imam al Baqir, considered to be the fifth infallible Imam by the Shia married Umm Farwah bint Qasim ibn Muhammad ibn Abi Bakr. It was from this union that his son, Jafar al Sadiq—considered the sixth infallible Imam by the Shia—was born.
Imam Jafar al Sadiq has a famous saying concerning this, which has been reported in a number of sources, the best of which is what Ibn ‘Inabah has recorded:
His (al Baqir’s) progeny continued from Abu ‘Abdullah Jafar al Sadiq ‘alayh al Salam only. His mother was Umm Farwah bint Qasim al faqih ibn Muhammad ibn Abi Bakr. Her mother was Asma’ bint ‘Abdul Rahman ibn Abi Bakr. This is why al Sadiq ‘alayh al Salam would say, “Abu Bakr fathered me twice,” and he would be referred to as Pillar of Nobility.
The meaning of “fathered me twice” is that Abu Bakr radiya Llahu ‘anhu is his grandfather through his mother both paternally and maternally: Paternally in that his mother is the daughter of Qasim ibn Muhammad ibn Abi Bakr, and maternally in that his mother is the daughter of Asma’ bint ‘Abdul Rahman ibn Abi Bakr.
It is also said that “fathered me twice” means that he is his biological (grand)father and spiritual (grand)father; since Imam Jafar rahimahu Llah gained knowledge from a number of teachers amongst whom is Qasim ibn Muhammad ibn Abi Bakr—who was one of the most senior Jurists of Madinah. Qasim ibn Muhammad himself was nurtured and brought up by Sayyidah Aisha al Siddiqah radiya Llahu ‘anha, from who he narrated and gained knowledge.
As for Umm Farwah radiya Llahu ‘anha the books are filled with accounts of her immense taqwa and piety. ‘Abbas al Qummi writes:
Jafar said, “My mother was amongst those who believed, was god-fearing, and a one who did abundant good, and verily Allah loves those who do good.
‘Ali ibn Hussain al Mas’udi said in Ithbat al Wasiyyah that Umm Farwah was the most pious woman of her age, she reports from ‘Ali bin Hussain ‘alayh al Salam…
The statement of Imam Ja’far al Sadiq radiya Llahu ‘anhu, “Abu Bakr fathered me twice,” is well-known and accepted; however, while browsing online I have found many prejudiced individuals denying the honourable Imam ever having made such a statement. I therefore thought it appropriate to list some of the sources which establish this statement, sources whose credibility and importance is beyond doubt.
Ibn ‘Inabah records:
His (al Baqir’s) progeny continued from Abu ‘Abdullah Jafar al Sadiq ‘alayh al Salam only. His mother was Umm Farwah bint Qasim al faqih ibn Muhamad ibn Abi Bakr. Her mother was Asma’ bint ‘Abdul Rahman ibn Abi Bakr. This is why al Sadiq ‘alayh al Salam would say, “Abu Bakr fathered me twice,” and he would be referred to as Pillar of Nobility.
‘Ali ibn ‘Isa writes:
Hafiz ‘Abdul ‘Aziz al Akhdar al Janabidhi said, “Abu ‘Abdullah Jafar ibn Muhammad ibn ‘Ali ibn Hussain ibn ‘Ali ibn Abi Talib… his mother was Umm Farwah, here name Quraybah bint Qasim ibn Muhammad ibn Abi Bakr al Siddiq radiya Llahu ‘anhu. Her mother was Asma’ bint ‘Abdul Rahman ibn Abi Bakr. This is why Jafar ‘alayh al Salam would say, “Abu Bakr fathered me twice.”
It appears in al Asili:
As for Abu ‘Abdullah Jafar ibn Muhammad al Sadiq ‘alayh al Salam… his mother and the mother of his brother ‘Abdullah was Umm Farwah bint Qasim ibn Muhammad ibn Abi Bakr. Her mother was Asma’ bint ‘Abdul Rahman ibn Abi Bakr. This is why Jafar ibn Muhammad ‘alayh al Salam would say, “Abu Bakr fathered me twice.”
The narrations mentioned above should be sufficient, with a number of great scholars referring to this statement.
The great grandson of Hassan ibn ‘Ali married Umm Salamah bint Muhammad ibn Talhah ibn ‘Ubaidullah ibn ‘Abdul Rahman ibn Abi Bakr al Siddiq. They were blessed with a son who they named ‘Abdullah.
The grandson of the fourth Imam Zayn al ‘Abidin married Kulthum bint Ismail ibn ‘Abdul Rahman ibn Qasim ibn Muhammad ibn Abi Bakr al Siddiq.
Mus’ab al Zubairi writes:
Ishaq ibn ‘Abdullah ibn ‘Ali ibn Hussain ibn ‘Ali ibn Abi Talib had a son named Yahya…and Khadijah bint Ishaq her mother was Kulthum bint Ismail ibn ‘Abdul Rahman ibn Qasim ibn Muhammad ibn Abi Bakr al Siddiq. Her uterine brother was Qasim ibn Ibrahim ibn Walid ibn Muhammad ibn Hisham ibn Ismail al Makhzumi.
 Surah al Furqan: 54
 Fath al Bari, 7/452, under the commentary of Hadith: 3729.
 Al Rahiq al Makhtum, pg. 480-481.
 This narration was mentioned by Ibn Hajar al ‘Asqalani in his commentary of Sahih al Bukhari, 7/452, in his commentary of Hadith: 3729. He said, “Al Hakim reported it under the virtues of ‘Ali and it has a corroborating narration from ‘Abdullah ibn ‘Umar. It is reported by al Tabarani in al Awsat with a weak chain.” The narration is da’if as it contains ‘Ammar ibn Saif.
 Surah al Nur: 26
 Sahih Muslim with the commentary of al Nawawi, # 2438, # 2439.
 It is reported that Hafsah bint ‘Abdul Rahman ibn Abi Bakr al Siddiq married Hussain ibn ‘Ali radiya Llahu ‘anhu, as mentioned by Ibn ‘Asakir, reporting on the authority of Ibn Sa’d from his Tabaqat. However, I am skeptical about this, which is why I have not mentioned it in the actual book.
 ‘Umdat al Talib, pg. 37.
 ‘Umdat al Talib, pg. 176.
 Muntaha al Amal fi Tawarikh al Nabi wa al Al, 2/161.
 ‘Umdat al Talib, pg. 176.
 Qouted as it appears in Kashf al Ghummah.
 Kashf al Ghummah, 2/347.
 Al Asili, pg. 149.
 Siblings who share the same mother but different father.
 Nasab Quraysh, pg. 65.