1. Question: If a husband had two wives die from childbirth why can he marry another one but if two sons die from bris milah his third son cannot have a bris?
A. The Shulchan Aruch (Even HaEzer 9:2) says that if a man had two wives die, that does not mean he can’t get married again. Even though if a woman had two husbands die, she isn’t allowed to remarry a third time because there is an assumption that she has bad mazel and any man she will marry will die, as stated in si’if 1 (See Beis Shmuel 9:1:1 there).
B. The father is “blamed” for the deaths of his sons and once two died we assume the third will die as well from circumcision.
Answer: We can assume that the father gave his sons “weak genes” so that they physically can’t handle the bris but that would not affect his wives as the Beis Shmuel says in si’if katan 7.