Restricted relations for Marriage

Marriage is forbidden between very close blood relations and between those individuals who were both breastfed by the same woman (see wet nurse). See also mahram for a fuller discussion of unmarriageable kin; Muslims are free to marry anyone not in these prohibited classes.
Abu Hurairah narrated:

The Prophet forbade that a woman should be married to a man along with her paternal aunt or with her maternal aunt (at the same time). Az-Zuhri (the sub-narrator) said: There is a similar order for the paternal aunt of the father of one's wife, for 'Ursa told me that 'Aisha said, "What is unlawful because of blood relations, is also unlawful because of the corresponding foster suckling relations."

—Sahih al-Bukhari, Volume 7, Book 62, Number 46

`Abd Allah ibn `Abbas narrated:

It was said to the Prophet, "Won't you marry the daughter of Hamza?" He said, "She is my foster niece (brother's daughter)."

—Volume 7, Book 62, Number 37