I can totally sympathise. I have just had enough of my MIL's snidey, bitchy, catty, critical comments after 9 years of keeping my mouth shut and not saying a word to her in retaliation. My MIL had an opportunity last year to apologise to me, she didn't take it. I tolerated her for another year but now I have had enough. DH phoned her a week ago and told her she has been behaving like a bully towards me, that she had completely and utterly overstepped the mark on many occasions and that I had had enough of her and would not be seeing her again for the forseeable future.
However, in my case the DC's have an established relationship with her and I can see she loves them dearly and treats them well so I am not stopping them from seeing her. Nor am I stopping DH from seeing her. But I will not be laying eyes on that woman for as long as possible.
I don't believe a leopard can change it's spots, she couldn't even keep her mouth shut for DH's sake (he thought she would be able to but it turns out I knew his mother better than he did and within months she was back to her old ways).
Like your MIL, mine is an nasty piece of work, full of bile, spite and venom and I cannot even bear the thought of laying eyes on her never mind talking to her or more likely, listening to her making snidey remarks to me which, if pulled up on, she tries to pass of as innocently tactless,
FIL is spineless too and deaf; he says the greatest blessing he has had in old age is to lose his hearing! I know why, you would have to be deaf to put up living with that woman for long.
I would say to you follow your gut instincts and stay away from her. Like others have said, if she shows you she has taken time out to reflect and think about how she has treated you in the past and can show you genuine remorse and apologise then you could perhaps reconsider but not otherwise.