SIL From Hell...Advice needed

That was the last straw for me (this girl has been nothing but a bitch to me since I met her 6 years ago) and I made a snarky comment to her along the lines of "its always nice to find out about major family news through Facebook". Well, that instigated a huge chain reaction of her sending me a rude email, me sending one back, her sending one back but addressed to my husband and not me and literally said something to the effect of "we had years of fun memories before your wife came into the picture", which made me furious. I then threatened to tell my husbands sister and her husband, all the horrible, nasty things she's said about them (she pretends to be best friends with his sister, she has a pet name for her....yet has said the most awful things about her and even told me she talked to her friend who worked for CPS about how they treat their children), because if theres one thing I can't stand its someone who is fake. Anyways, threatening that was not my finest moment but at that point, I didn't care. Anyways, then my husband and his brother talked and it basically died down. I told her that the situation had spiraled out of control and if she wanted to talk to resolve the situation, she could call or email me. She never did. 

Fast forward to this spring, I sent her an email saying her maternity pictures were really pretty, she looked great, and that I was sorry for what had happened last year and that I hoped that for our husbands sakes and their relationship, that we could eventually talk and move past it. She completely ignored the email and I never got a response.

Fast forward, seven months later to now. About 5 days ago, I messaged her and said that I wanted to apologize for everything that had happened and that I know we've never had a close relationship and that it was partially my fault and that I was sorry and also that I felt badly for some of the things I said to her in the heat of the moment. She read it and ignored it. 

I have to see this girl over the holidays and it literally make me cringe thinking about it. She will kiss my ass in public because she can only be a bitch on the internet and I can't pretend to like someone who acts like this. I've done everything I can and she is just intent on ruining the relationship that my husband and his brother have. What would you guys do?