You have my empathies. My older sister wrote me off in 2004 for dressing my newborn in a "Bush/Cheney" onesie. I always knew she was bleeding-heart lib, but she just wrote me and the rest of my direct family out of her life. It's heart breaking, family shattering stuff in the long run. I feel for your circumstances.
I did the olive branch a few times and it's not possible any more. It's now a sister I once had a long time ago and nothing more. Hoping you find peace.