Just getting to this now.
Zoso, here's what troubles me. In my sister's mind, I'm sure she's constructed some narrative where SHE has to draw the boundaries with my mom, the judgmental and unforgiving one. So now I'm just as guilty of doing that if I draw the boundaries with my sister. And being a hypocrite to boot.
You have no control over what your sister thinks or doesn't think. Zero. Nada. Zilch. What do you have control over is your choices and who you allow to be in your life. And if you don't want her in your life, that's your right.