Is it permissible for the father of the child suckled by the mother – who has become a brother to her children – to marry one of their sisters?

The father of the child may marry his deceased wife's sister, unless there is a proven suckling relationship between them. She is not unlawful for him to marry just because she is the sister of his child through suckling, because suckling does not establish unlawfulness in this specific case. The suckling child is considered the child of the milk-giver, and it is the descendants of this milk-giver who become unlawful to marry, similar to those prohibited by lineage.