What is the ruling on this marriage, and is it invalid? And what should be done with the sister who married a supporter of the Nusayri sect under wartime conditions? And is boycotting her considered a severing of family ties?

If your father completed your sister’s marriage contract to a Muslim man who is committed to prayer, in the presence of two Muslim witnesses, then the marriage is valid. It is not permissible for you to sever ties of kinship with your sister, even if she has erred. If it becomes clear that her husband is not Muslim, then you must strive to rescue your sister from this marriage, and this emphasizes your need to maintain ties with her.