I would like to offer an insight. Your family sounds as if its maternal center is a narcissist…not simply a garden variety but one with a narcissistic personality disorder. The characteristics you shared are textbook NPD. As you described her, your sister is a likely candidate as well.

The good in the message comes with your instincts as to how best manage this more than difficult conundrum. In the terms of the counter measures used in the world of a narcissist personality disorder comes “gray rocking”. It is at its roots simply being distant no matter their proximity. It is detachment emotionally without a formal declaration. This tactic is typical in situations such as yours where family comes at a high price while nevertheless is always there.

Good luck and safe journey.

Eric