The Breast-feeding of an Adult Has no Effect
Question :
I have heard from some people that a man being breast-fed by his wife is not forbidden, and this has caused me some misgivings. How can his wife be his mother through breast-feeding and not be forbidden? I request clarification.
Answer:
The breast-feeding of an adult has no effect, because the breast-feeding which has an effect is that which takes place five times or more during the first two years and before weaning. As for breast-feeding an adult, it has no effect, and therefore, should it happen that someone was breast-fed by his wife, or he drank her milk, he would not be her son.
Source:
Ibn 'Uthaimin
Fatawa Islamiyah, Vol. 6 Pages 142-143