I don’t think that is true. A lot of us have close and/or romantic relationships with sufferers. My partner has severe Combat PTSD, and while he can be symptomatic and horrid at times, he is still “him.”
Doubt it. She’d still find something to lash out about, and you wouldn’t have your sister.
She’d be suffering regardless. Your sister may have triggered something with her PTSD, but it is NOT you or your sister’s fault. Cutting contact isn’t going to make her better.
You mom is still your mom, you’re just seeing the symptomatic/triggered side. She may get better, she may get worse, but that is all on her and nobody or nothing else.