You sound like you'll be a great stepmom.

However, you'll also be the dad's wife. That means he no longer should receive any invitation just to him unless it's a night out with the boys. It should be to "Husband and Wife". He deserves that respect. And so do you.

If nothing else, you don't want to have implicate their mother's feelings when you explain to them (or everyone else) why you don't attend certain family events. That just draws attention to the matter. And some kids may not even "get" why you don't go. If the mom doesn't want to see you and doesn't have a civil bone in her body, that's her problem.

Social etiquette exists for good reason :)