Will a girl be sinful if she severs ties with her brother after he attempted to molest her, and her parents forbade her from doing so and refused to believe her, knowing that her husband supported her decision to sever ties? And what is the ruling of Sharia on that?

Harassment is one of the greatest evils. The fundamental principle is the obligation to maintain ties of kinship and the prohibition of severing them, except for a legitimate Sharia interest. This includes shunning a transgressor if there is a benefit in doing so. If the brother persists in this transgression, then shunning him is permissible, and there is no sin in boycotting him. However, if he ceases this behavior, then it is not permissible to sever ties with him, and the estrangement ends with salutations and similar actions.