You need a social worker and a counselor.
Good advice. This situation has festered for a long time and it's clearly past speding time to try to find a solution or to change the interpersonal dynamic.
A session or three with a skilled counsellor may very well provide you with how best to deal with this for your benefit ... to protect yourself ... perhaps even develop an effective way for you to influence it (which may involve being on the sidelines and out of the action for a bit.)
Best of luck and Cheers!