Will the questioner be held accountable for severing ties with his sister who caused problems, was abusive towards their mother and the questioner, and did not attempt to contact their parents?

Disobeying parents is one of the greatest sins. However, it does not prevent maintaining ties with a disobedient sister, as the Prophet, peace and blessings be upon him, commanded maintaining ties with his polytheist mother. You must maintain ties with your sister, advise her, warn her of the consequences of disobedience, and pray for her guidance. If she does not heed the advice, and if boycotting her serves as a deterrent to her disobedience, then there is no sin in boycotting her. In fact, she must be shunned if that leads to her rectification.