Look, you don't like her or want to be around her, or vice versa, but she does talk to your husband and you have a great relationship with your dad.
You have to work so it seems if your husband wants to go over while you do that it wouldn't be so bad. I understand you might feel a little betrayed, but it's not like SHE said, come over and DON'T bring your wife. At least not in this instance. You aren't available.
You have a right to your feelings, but maybe this is a bridge that can begin being built. She's had no relationship with your kids or even seen your second son. Maybe this is the start of changing things because as you said yourself, her behavior has hurt your father. If she does what's been typical and leaves when your husband and kids are there, what is the harm done? Your husband and boys will spend time with your dad, which you are fine with.
It might not be as bad as you think, is all I'm saying and 3 years is a long time not to see or speak to someone you're related to whether either of you like it or not.
If you truly can't stand her, then be glad you're not going.
If you truly hope you can mend some fences, maybe let your husband reach out and try to arrange a time you can all visit.
I have lost so many people recently, in fact, I'm going to a funeral tomorrow. Sometimes life is too short for things that start out on the wrong foot to just get dragged out. Especially where family is concerned.
I can't say I exactly adored my step mother and she did things that really ticked me off, but I most definitely tolerated her for my dad's sake. He was married to her way longer than he was married to my own mother and I tried to keep my feelings about her irritating behavior to myself out of love for my dad. You find ways around things. I never, ever would have kept my kids from my father because of my step mother. She actually was thoughtful toward my children. She would not have harmed them for a second.
I think you should let the kids go and like I said, if she leaves, what difference does it make? But, maybe there could be an olive branch involved in this scenario somehow.
That wouldn't be such a bad thing.
This is just my opinion and I really wish you the best and hope you can find a way to work this out so you are all happy.