If two sisters breast feed each other's children will their fathers become Mahram to the children. Also whether daughters and sons of each sisters become Mahram?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

If both sisters suckle each other's children then the husband of the suckling-mother becomes the suckling-father for the breastfed child only, not for his other brothers. So, the rulings of an unmarriageable relative such as looking at unmarriageable women, staying alone with them and prohibition of marriage, etc. also applies to the foster-child. The foster-father of the child does not become a foster-father for the other children who have not suckled from his wife.

The child who suckles from his aunt becomes foster-brother for all the children of his aunt. The brothers who do not suckle from their aunt, neither become foster brothers of their cousins nor do they become foster-children for their aunt's husband.

It is written in 'Az-Zaad' a book of Hanbali school: 'Whenever a woman suckles a child he becomes her child as far as the rulings of marriage, looking at her, and being alone with her, are concerned. This child also becomes a child for the man, and the unmarriageable relatives of the foster-parents also become unmarriageable for the foster-child. This ruling applies to the foster-child only; it does not cover even his original parents and children.'

Know that the above rulings are effective only if the child suckles for five times.

Allaah Knows best.