H.W.
Oh my gosh, you sound like my mother. Bitter, angry, no one can do anything right, her way... and she tried to poison the kids against my dad's second wife and dad himself.
I'm going to give it to you short and sweet: my sister and I don't have a relationship with her any more. Wanna go down that road? because lady, you are already driving down it. Parental alienation can really backfire.
As others have suggested:
1/ Stay out of their marriage and stop trying to play the "just you wait and see, you'll be pissed and miserable like me" thing with his wife.
2/ Accept reality. She's their stepmom and you have a custodial order saying the kids may be at their father's house. I spent plenty of time with my stepdad when mom was on trips. Ditto my stepmom when my dad was at National Guard or out fighting fires with them.
3/Do your job as a mom, which is to provide containment, peace and stability for your children. Your kids are better served with YOU having a good attitude about this. YOU are the example.
4/ Stop being mad and complaining, grow up. If you are constantly looking outwardly to 'fix' things, you miss your first best asset-- fix yourself and your own attitude first. Goes a much longer way than being snotty and mean. Go get some counseling if you need to-- learn to accept the reality YOU helped to create and learn how to interact with people in a healthy way.