Please understand that the kids probably don’t hate you as a person. They hate the disruption in their lives, and their mother has made you the scapegoat. The kids are in a terrible position. They know their mother is angry. They may think that if they show affection for you, their mother will reject them.
You don’t mention where your husband is in all this. You need his help. You can’t change their mother. But you and their father can sit down with the kids and tell them that you understand the position they are in and wish you could make it different. Don’t trash their mom. Just remind them that there are good, adult reasons why their parents divorced that don’t involve them. Emphasize that their dad didn’t divorce them and that the two of you are doing the best you can to make a good life for everyone. You can make it clear that you have no intention of trying to replace their mom but that you all could find a way for you to be an adult friend.
Let their dad take the lead in this conversation. The kids need to hear from him that he loves them and respects their feelings but that they still have to treat you with respect. They don’t have to love you, but they do have to be civil. Then he has to follow through. If the kids treat you badly, he needs to remind them that it’s simply not okay to make you into the bad guy. Meanwhile, you need to acknowledge their feelings but not get upset by them. You can say something like, “I know this is hard for you but treating me badly isn’t going to help the situation. Let’s figure out how to be friendly even if we can’t be friends.” Then do what you can to change the subject. If they won’t let you, leave the room if you can, say something like, “I can see that you are too upset to listen so I’m going to take a time out. Let me know if there’s a time you want to talk about things more calmly.”
Step parenting is one of the hardest jobs in the world. I applaud you for your efforts. Your parenting is working with your birth-kids so you are obviously doing something right. Hopefully you and your husband can get on the same page and pull his other kids into your family.
I wish you well.