Is it a sin for a woman to sit with her sisters' husbands in her father's house, after she used to seclude herself from them, due to her husband's disagreement with her regarding the issue of intermingling?

It is not permissible for you to sit in family gatherings where Sharia controls are not observed, even if you are veiled, because it is not permissible to be present in a place where there is evil unless it is for the purpose of condemning it. As for your husband sitting with your brother when he and his wife visit, this is a required act of honoring the guest. This way, your husband avoids sitting with your brother's wife. If your brother wishes to sit with you, he can visit you alone, or it can take place at your father's house.