Running around in one's bra and panties is like running around in a bikini...unless they are see-through.
I understand your frustration...but think you might be a little too close to the situation.
I don't know how you and your ex divorced, how long it's been or whether anything has been amicable during this time. I don't know if you are jealous of new wife or not.
Is your daughter OFFENDED that her step-mom is running around in bra and panties or just not comfortable? Or is she saying this to get a rise out of you - which it did and to stir up trouble? Kids can be VERY manipulative when it comes their parents...
I have two boys - I run around in my bra and undies (granted, i don't wear see through bras nor do I wear thong undies when I am running around)...but that's ME. I don't do this while we have friends over and I don't do this just for the heck of it.
While it's YOUR opinion that what she is doing is wrong - before you start throwing allegations out there or at her and your ex - you need to think of WHAT you are trying to accomplish - if you go at this like a bull in a china closet - you aren't going to get anywhere but more headache and trouble.
1. IF you have a good relationship with your ex - go to ex and say "John - Susie is not comfortable with Sharon running around in her bra and panties - can you please talk to her about this?"
2. IF you have a crappy relationship with your ex's new wife - then you need to be an adult and show your daughter that even though you don't like someone - you can still be NICE AND get thing accomplished by being NICE...."John, Susie is not comfortable with Sharon walking around the house in bra and panties...I would appreciate it if Sharon would NOT do this while Susie is with you."
3. IF you have a good relationship with your ex and his new wife - call her and ask to meet for coffee - let her know that Susie has expressed her feelings and is uncomfortable with her walking around the house in bra and panties and before it gets out of hand - is there some way we can compromise on this?
Anger isn't going to work. Yelling allegations will not work - it will cost money to hire a lawyer and just stir up trouble. It doesn't appear that the step mom is flaunting her body to or touching your daughter - so this is a PERSONAL choice. You are ALWAYS going to be tied to this man - you have a child together. You two, although divorced, need to get on the same page and be adults. Let go of the anger, resentment and overall bad-ju-ju and keep it simple. Life works sooo much better that way. From your post - you are VERY angry over something that is VERY SMALL (in my opinion). You need to take a deep breath, take a step back and be nice. Be the adult. You don't like what's going on in the house - COMMUNICATE. Your idea of unstable and someone else's idea of unstable may not be the same. So when talking with your ex and his new wife "IT IS MY OPINION that Sharon walking around in bra and panties is inappropriate. It is ALSO the opinion of OUR DAUGHTER's that it is inappropriate. We need to find some common ground here..."
I wish you the best in letting go of your anger, hurt and resentment. I hope that your daughter is NOT trying to manipulate you and get a rise out of you.