I have a mother-in-law AND a sister-in-law whom I haven't spoken to in years, and probably for some of the same reasons. Forgiving does NOT mean forgetting, nor does it mean posting a sign on your backside saying "Please, kick me again". It just means getting on with your life and doing your thing, and they can do theirs.
Meanwhile, if it ever happens that we must be together in the same place, I'd merely be coolly civil, not inviting conversation but not cutting them dead, so to speak, by ignoring them entirely. A nod in their direction will suffice, and if they want to speak further, I'd just make certain that I'm not available.
And yes, congratulations to your hubby!! I'm sure he doesn't want the enmity between you and his mother to spoil the day, either.......he's got more important things going on.