Because Allah, the Exalted, says: «حولين كاملين لمن أراد أن يتم الرضاعة‏» "Complete two years for the one who wishes to complete the period of suckling, and if the suckling is less, then prohibition is established, and if it is more, then as well."

Narrated `Aisha: that the Prophet entered upon her while a man was sitting with her. Signs of answer seemed to appear on his face as if he disliked that. She said, "Here is my (foster) brother." He said, "Be sure as to who is your foster brother, for foster suckling relationship is established only when milk is the only food of the child."

Narrated Aisha: that Aflah the brother of Abu Al-Qu'ais, her foster uncle, came, asking permission to enter upon her after the Verse of Al-Hijab (the use of veils by women) was revealed. `Aisha added: I did not allow him to enter, but when Allah's Apostle came, I told him what I had done, and he ordered me to give him permission.

Narrated `Uqba bin Al-Harith: I married a woman and then a black lady came to us and said, "I have suckled you both (you and your wife)." So I came to the Prophet and said, "I married so-and-so and then a black lady came to us and said to me, 'I have suckled both of you.' But I think she is a liar." The Prophet turned his face away from me and I moved to face his face, and said, "She is a liar." The Prophet said, "How (can you keep her as your wife) when that lady has said that she has suckled both of you? So abandon (i.e., divorce) her (your wife).

And Allah the Exalted has mentioned them (in Surah An-Nisa) «حرمت عليكم أمهاتكم وبناتكم وأخواتكم وعماتكم وخالاتكم وبنات الأخ وبنات الأخت‏» : "Forbidden to you (for marriage) are your mothers, your daughters, your sisters, your paternal aunts, your maternal aunts, your brother's daughters, your sister's daughters. Surely Allah is All-Knowing, All-Wise." Anas bin Malik (may Allah be pleased with him) said «والمحصنات من النساء‏» that by "married women" is meant those who are free, they are also forbidden. And «وما ملكت أيمانكم» means that if a slave woman is married to a slave, she can be taken from the slave (i.e., divorced) and made one's own wife. And Allah has also said «ولا تنكحوا المشركات حتى يؤمن‏» : "And do not marry polytheist women until they believe." And Ibn Abbas (may Allah be pleased with them both) said: It is forbidden to marry a fifth woman while having four wives, just as it is forbidden to marry one's mother, daughter, or sister.

Ibn 'Abbas further said, "Seven types of marriages are unlawful because of blood relations, and seven because of marriage relations." Then Ibn 'Abbas recited the Verse: "Forbidden for you (for marriages) are your mothers..." (4:23). 'Abdullah bin Ja'far married the daughter and wife of 'Ali at the same time (they were step-daughter and mother). Ibn Sirin said, "There is no harm in that." But Al-Hasan Al-Basri disapproved of it at first, but then said that there was no harm in it. Al-Hasan bin Al-Hasan bin 'Ali married two of his cousins in one night. Ja'far bin Zaid disapproved of that because of it would bring hatred (between the two cousins), but it is not unlawful, as Allah said, "Lawful to you are all others [beyond those (mentioned)] . (4:24).Ibn 'Abbas said: "If somebody commits illegal sexual intercourse with his wife's sister, his wife does not become unlawful for him."And narrated Abu Ja'far, "If a person commits homosexuality with a boy, then the mother of that boy is unlawful for him to marry."Narrated Ibn 'Abbas, "If one commits illegal sexual intercourse with his mother in law, then his married relation to his wife does not become unlawful." Abu Nasr reported to have said that Ibn 'Abbas in the above case, regarded his marital relation to his wife unlawful, but Abu Nasr is not known well for hearing Hadith from Ibn 'Abbas.Imran bin Hussain, Jabir b. Zaid, Al-Hasan and some other Iraqi's, are reported to have judged that his marital relations to his wife would be unlawful. In the above case Abu Hurairah said, "The marital relation to one's wife does not become unlawful except if one as had sexual intercourse (with her mother)." Ibn Al-Musaiyab, 'Urwa, and Az-Zuhri allows such person to keep his wife. 'Ali said, "His marital relations to his wife does not become unlawful."

And forbidden to you are your wives’ daughters (those whom they bring from other husbands) whom you rear, when you have consummated marriage with those wives. And Ibn Abbas (may Allah be pleased with them both) said that the words «دخول» , «مسيس» , and «ماس» all mean intercourse. And the explanation of the statement that the offspring of the wife (such as granddaughters or daughters’ daughters) are also forbidden, because the Prophet (peace be upon him) said to Umm Habibah (may Allah be pleased with her):“Do not present your daughters and sisters to me (for marriage).”So among daughters are included the son’s daughter (granddaughter) and the daughter’s daughter (daughter’s daughter), all of them. And likewise, among daughters-in-law are included the son’s wife (grandson’s wife), and among daughters are included the son’s daughters (granddaughters) and daughters’ daughters (granddaughters through daughters), all are included. And the wife’s daughter is always a stepdaughter, whether she is raised by the husband or by someone else, in every way forbidden. And the Prophet (peace be upon him) gave his stepdaughter (Zaynab, who was the daughter of Abu Salamah) to another person (Nawfal al-Ashja‘i) to be raised, and the Prophet (peace be upon him) called his grandson Hasan (may Allah be pleased with him) his son.

Narrated Um Habiba: I said, "O Allah's Apostle! Do you like to have (my sister) the daughter of Abu Sufyan?" The Prophet said, "What shall I do (with her)?" I said, "Marry her." He said, "Do you like that?" I said, "(Yes), for even now I am not your only wife, so I like that my sister should share you with me." He said, "She is not lawful for me (to marry)." I said, "We have heard that you want to marry." He said, "The daughter of Um Salama?" I said, "Yes." He said, "Even if she were not my stepdaughter, she should be unlawful for me to marry, for Thuwaiba suckled me and her father (Abu Salama). So you should neither present your daughters, nor your sisters, to me."

Narrated Um Habiba: I said, "O Allah's Apostle! Marry my sister, the daughter of Abu Sufyan." He said, "Do you like that?" I said, "Yes, for even now I am not your only wife; and the most beloved person to share the good with me is my sister." The Prophet said, "But that is not lawful for me (i.e., to be married to two sisters at a time.)" I said, "O Allah's Apostle! By Allah, we have heard that you want to marry Durra, the daughter of Abu Salama." He said, "You mean the daughter of Um Salama?" I said, "Yes." He said, "By Allah! Even if she were not my stepdaughter, she would not be lawful for me to marry, for she is my foster niece, for Thuwaiba has suckled me and Abu Salama; so you should neither present your daughters, nor your sisters to me."

Narrated Jabir: Allah's Apostle forbade that a woman should be married to man along with her paternal or maternal aunt.