I can’t tell from your post if he is still with his mistress or if it’s over, but the real issue here is between you and your husband — not her.
I’m not sure what the rough patch in your 10 marriage was about, or if it’s worked through or not, but the two of you have to decide if you’re going to stay together or not and how. That he’s having his mistress (current or ex) at these family events is not a sign that he values your marriage. In fact, it’s a red flag to you that he’s going to keep his mistress in his life and that your feelings about that relationship are less important than his having there.
The big question is whether you’re okay with this or not. If you are, then try to focus on things with the two of you. Understand that his brother is not supportive of your marriage. If you are not okay with this, then you have an option to leave the marriage. He would have to be pretty dense to know that this is insulting and disrespectful to you, so I think the ball is really in your court. I’m sorry you’re going through this, but it’s time for some tough decisions.