Is it permissible for a son to marry a woman who is an innovator (mubtadi‘ah)? And would their children be legitimate? And is it permissible to ostracize them due to the problems that arose from this marriage?

We have previously explained in detail the ruling on marriage to someone who differs in belief, differentiating between a heresy that amounts to disbelief and one that does not, and based on that, the ruling on the marriage is determined. If the marriage is deemed invalid, his children from this woman are still attributed to him if he believed his marriage to her was valid. It is not permissible for his brothers to cut ties with him; rather, it is their right to advise him kindly and explain the truth to him with goodness. If he repents and returns to Allah, then that is good. Otherwise, they may ostracize him if they hope that ostracism will benefit him.