I totally, completely, 100% understand why you don't want her there. I feel for you and sympathise. The situation sucks and you have every right to be pissed off, miserable, and hoping they would move to Timbuktu.
But as a child of divorced parents married to someone with divorced parents please read, re-read, re-read and re-read OldDaddy's message. This is exactly how it will end up if you go down the avoiding each other path. It will be pure hell. My MIL did it, and I can tell you that even now (thirty years later!) there are so many things she misses out on in her children's and grandchildren's lives because the whole family has got used to inviting one or other. I think by now she wouldn't mind sometimes being in the same place, but the "which one shall we ask" habit has been set. And to be honest, often her children avoid their home town at times like Christmas to get away from the "who are you going to choose" pressure. All the time I see my DH in distress because he is so pulled between them by his mother. The only argument I have ever had with any of my ILs was with a BIL over a "who shall we choose" dilemma. It is horrible.
My own mother did not choose this path when my father ran off. She has had a much happier outcome in terms of involvement in her children's and grandchildren's lives. She very early on took a pragmatic decision that she would have to tolerate her ex and his OW for the sake of her own relationship with her own children - by not ever making us choose between them. I know this often hurt like hell and was hugely difficult to manage. But she would tell you it was worth it, and as her child I agree.
So horrible, difficult and unfair as it may be, I do think that the long game, for you, means sucking it up. Have you got a friend who would go and stand with you? Big takeaway coffee each and plenty of gossip?