Am I entitled to refuse my husband's proposal to a girl who caused the ruin of my home, and would I be sinful for that, even though I gave him the freedom to marry?

It is the wife's right to refuse to go to her husband's engagement to another woman, and she does not sin by disobeying him in that, for her obedience to him is not absolute. The husband must repent of his relationship with that foreign girl, because it is not permissible outside the framework of a valid marriage.