What did you say to his sister? I notice you left that out, but it was enough to make her cry. That sounds kind of bad, to be honest.
I think you should stop gossiping with your husband’s biological father’s new gf or anyone else who happens to tell you what someone else said about you, (no one spreading stuff like that has good intentions) they all definitely sound like you think they are a bunch of jerks. Try to put aside all the very personal feelings you are feeling. No matter what happened, you still insulted his sister, her daughter and everyone is still very offended by this. Have you apologized? What kind of relationship does your husband want with his family? What kind of relationship do you want with them? Talk about this with him. He is the person who needs to know what you want so he can deal with his family. To me, it’s sounds like his mother was trying to include you by inviting you to be involved in things, again if you and your husband want something different, you need to talk about it together (communicate) and decide what you want. If he loves seeing his family this much and wants nothing to change, then that’s where you need to decide what you want, and how you can make do with that the best you can. Talk to him and figure out how to mend this, or move on.