You may benefit from individual counseling with a counselor expert in the dynamics of dysfunctional families, who also does not believe in family members sticking together at all costs.
For me, picking apart the interactions, discovering the motives behind her treatment of me, and realizing I am/was well within my rights to put boundaries in place to block her were helpful. It wasn't about me at all, it was always about her getting ego supply. She was very insidious and sneaky with left handed compliments and spending freely which was nothing more than bait.
If your sister is assaulting you, call the police. Seriously, she is banking on the fact she doesn't think you have the backbone to do that.
May I ask how old you are? Sometimes people get to a certain age (mid forties usually) and just do not put up with that crap anymore. You do not have to wait until then to draw the line!
Are you still living in the same town as your sister? Physical distance helps immeasurably.
Have you considered stepping away from FB? I'm not on FB (never have been) so don't know the ins and outs of hiding filters or what not, but if you know your sister is on there, why take the chance of stumbling across her?
I am very glad for you that you have your mother's support. That means so much. Especially since she can see your sister's behavior for the abuse that it is-she is what is called an enlightened witness. This is very important in validating you and your perspective.
Have a look at the Stately Homes thread on this board (But We Took You to Stately Homes-support for survivors of dysfunctional families) and post more of your story there if you like. (I'm not sure how to link it
)