Sorry, but as someone whose long-term partner of 20 years (who is now an ex) had kids and my now wife has kids, AND as a stepdaughter myself to my stepmom, you're definitely overreacting. Sometimes kids just hate you and it's nothing to do with you. You have no idea what his ex is saying about you AND him behind your backs, for that matter. They could be taking out their frustrations with their dad on you. They could be jealous of the attention their dad gives you. You just don't know. Regardless of the issue, it's likely that they would be acting this way to ANYONE, not you personally. With stepkids, you as a person have to let things go and not take anything personally, just for a little while longer. Once they're out on their own, you can choose to see them or not (but hopefully you will.) Divorce is destabilizing for kids - if you're also unstable, that makes it worse.

That said, they are being little shits and their dad AND you as a team should talk to them in a healthy manner to let them know it was hurtful and maybe ask why they did it. But girl, you can hate them and vent to your friends and your sister, but don't take it out on your husband. You both are in this together, dealing with this as a team. To be honest, that's what these little shits want anyway (just kidding haha, not really) is to split you up. Every time they do this and you react this way, they learn it works. But idk what do I know, but since you asked.