I found this in a book called "The Lawful and the Prohibited in Islam" by
Yusuf Al-Qaradawi.

"It is unlawful for a Muslim to marry a woman who has suckled him during
infancy, for suckling makes her like his real mother, since milk has gone
into the making of his flesh and bones.  Nursing consciously or
unconsciously produces feelings of motherhood in a woman and of kinship in a
child, and although these feelings might seem to disappear as the child
grows and becomes a man, they remain hidden in the unconscious.

However, the prohibition of marriage based on fosterage is effective only if
the suckling occured before the time of weaning; that is, when milk was the
primary source of food.  Another condition is that the child has suckled his
fill on five separate occasions, a fill being defined as when the child
leaves off suckling of his own accord.

Just as a woman becomes a mother to a child by virtue of suckling, likewise
her daughters become his sisters, her sisters his aunts, and so on.  Thus it
is forbidden to marry the foster-sisters, foster-aunts, and foster-nieces."

Lisa Haider