Ignore... At least that's what I do. I HATE hearing aobut the beastie. But i find I don't hear any of the important stuff if I shut them down... Plus a lot of people on here seem to have shut it down, and then gotten some whiplash of the skid talking about it 2x as much just to be a brat...

to be told that both options are okay. It sounds like mom was more than "surprised" that SD wears underwear to bed and that it was almost a criticism or like she was doing something wrong by wearing panties. Girls that age are very sensitive to criticisms or even perceived criticisms, especially from mom. 

I would ignore for the most part but when it is a body  issue I would address it and tell SD that she should do what is most comfortable for her and that during her period underwear is probably best because of leakage and that properly fitting underwear shouldn't leave marks and that she is too young to worry about marks anyway. 

I never in my life slept in underwear unless on a period. Why wear underwear to bed? BUT it’s not something I would discuss with people. If I was you I’d not tell SD (or hint) that somehow sleeping with no underwear is wrong (just because you wear undewear to bed it doesn’t mean everyone does or should) but I’d tell her not to discuss adults’ personal business.  

yeah and dirty skids butts leave marks too

to get a response out of you.  

Confuse her by responding "I wonder what the neighbour says about that"  "I wonder what the guy in a blue suit I once saw says about that "   "I wonder what the lady that was in front of me at the checkout says"  blah blah.

In some cases if you ignore it, the behavior will escalate and if you say "Frankly, I dont CARE what the BM says" will cause you to reaffirm the reality that you despise the BM; further paving the way for SD to get under your skin.

Distract and bedazzle her with BS.  I also like the approach that it is personal and shouldn't be discussed.

I believe it is all about getting rise out of us.  In fact, I know it is with my skids, especially the middle daugher.  She ALWAYS brings her mom up, especially when we are all sitting at dinner together.  If I talk about my garden it's "oh well, my mom's garden...blah blah blah".  If I talk about our animals it's "oh my mom's cats....blah blah blah".  Ugh!  I could care less about her mother and she knows it!  Sometimes I want to say if your mom is so great, just stay with her because you certainly don't pay any attention to your father!  But, that would be totally catty and un-adult-like so I hold it in. 

I loved your comeback myself.  I would have said the same thing....or something similar. 

I don't think it is that personal. I think kids just like to talk about their parents and test whether it is okay, especially if they aren't allowed to at the other parent's house. I would just get over it and allow her to speak freely, but in the specific case, I would remind her that sometimes other people's private business should stay private and that BM may be embarrassed that SD told you guys that. 

or you could torture her with endless questions.  "My BM says you shouldn't wear underwear at night..."

REALLY?!  What kind of underwear does she like?

Do they have patterns?  I prefer plain black

Ohhhhhhh does she like thongs or full briefs.  Personally I like the boyfriend shorts style

I heard they had a sale on underwear at Fancy Store.  Does BM know that?  You should message and tell her?

Which underwear do you prefer SD?  OMG lets go on the xyz website and look at all the pretty ones!!!!

Bet she never mentions anything about what BM says again. 

I would ask her if she's capable of thinking for herself.  Teenagers hate to have their pride and independence challenged.  I would repeat it every time, like a broken record.