First of all, stop dwelling on past mistakes and any misgivings you have about how you conducted yourself. You are human and not perfect. You now have awareness of what was going on with you, so just use it as good information. Stop beating yourself up.
Second, you need to really sit down and talk with Sundance about how uncomfortable you are with his lying to her and painting a bad picture of you. Make sure he knows how important it is to you that you and she meet before their relationship proceeds. This is important! He knows Butch -- why shouldn't you know her?
While it may be true that going to her family event might simply be something rather casual and easy for her, to many people that would be a big deal. He's got to get honest with her. She would feel like total shit introducing him to her family with a certain idea of who he is and what his marriage is like, only to find out later that he lied to her about it.
Keep the lines of communication pen with him and ask him what he wants and needs to repair what's been happening between you!