Is my continued boycott of my sister, who crossed all bounds by insulting my deceased father and my mother who raised and married her, considered a sin, especially after all the severe abuse she inflicted upon us and our parents?

This sister is abusive and disobedient to her father, and severs ties with her relatives. The general rule is that you should not shun her unless shunning helps prevent her harm, or to deter her from evil deeds. The priority is to consider the religiously sound interest. If you predominantly believe that shunning her will bring her back to what is right, then shun her. But if the best course of action for her is to maintain ties with advice, then maintain ties with her and advise her.