If her parents aren't going to guide her there, I don't think there is much you can do

Been there done that with my YSD, she went for super dramatic, super seductive (tart-wear as DH used to call it )

She would change if DH told her to cover up or put something else on, but, otherwise she dressed very inappropriately and it was mostly attention-seeking behavior

One thing I did do, is on the occasions she was dressed nicely (and decently) I would make a point to compliment her on how nice she looked, when she dressed like a tart (I'm being polite here) I didn't pay much attention

She is now in her 30's, and still dresses really inappropriately on occasion. I Just think it's all about attention, and if they receive attention due to other things maybe they can be encouraged to focus more on that, but otherwise, the step-parent doesn't usually have much say and is probably expected to just butt out...

You can't care more than her parents. If dad lets her dress like a hooker than so be it. If it really bothers you, then don't go out in public with her. Everytime she wants you to take her somewhere while she's wearing slut wear, take a long look at her clothes and decline. If she or dad press the issue then let them know that you don't want people to think you agree with how she dresses and that you will join them when she's dressing more modestly. Maybe she'll change or maybe she won't. But either way, not your problem.

I agree with what's already been said - ie leave it alone. I used to have this same thing with younger SD posting inappropriate stuff on the internet. Often specifically sexual. I tried to tell DH this was going to get her in deep water some day. Neither he nor NPD BM seemed to be bothered, so in the end I stopped looking, or drawing his attention to it. Now at age 20 she seems to have become a lot more circumspect, thank goodness.

This is not a young child. This is a mid teen young woman. About the only thing you can do is what your DH is doing and that is not allow her to leave the house in inappropriate attire. For sure if she is going anywhere publically with you then you can dictate that she will not go with you unless appropriately dressed.

Lather, rinse, repeat. She will either catch a clue and develop some class or not. Her choice. She will learn eventually that others treat her in accordance to how she presents herself. If she presents herself as a cheap harlot... then that is how they will treat her. If she presenters herself with some style, character, and class... then that is how others will treat her.

The same applies to young men. If they present themselves as gutter punks they will be treated as gutter punks. If they present themselves with some style, character, and class they will be treated accordingly.

Pretty simple.

You could try the direct approach by complimenting her when she wears something classy enough to cover her assets. Take that chance to say "it's so much better than everyone feeling creeped out by your private parts being on show". I only say this because I witnessed this happen to a friend's daughter years ago and it was a lightbulb moment for her. She used to wear skirts that risked exposing all the detail and it was cringeworthy.

Nothing you can do about it, DH shuts you down and deals with it in his own way.... BM is not around...
You have tried, you talked to her everything and she ignores you... fine let her look the way she wants

I'm done telling Aergia to dress appropriately, all I do not ... if she's dressed like a slut and she has to go out with us (family event or what ever) I simply say, you will not be going with me dressed like that... move ...
SO knows better of saying anything....

If she hangs with her friends and go out with them - I say nothing....

Or... take all of the clothes you deem inappropriate to GoodWill.  End of inappropriate dress issue.  But,  get ready for WWIII.

Since my SD refuses to listen to her BD when he tells her to change. She has screamed in his face "my body my choice". She gets the clothes from her mother or cuts up clothes we have bought her. My new approach is if she is not dressed appropriately when we are going some where. I politely tell her she isn't coming and can stay home.  Her body her choice,  my outing my choice. 

Your situation isn`t as bad as you think it is. She's not your own daughter, so you don't have to put that kind of pressure on her. There's a reason she dresses so silly. She wants attention. Take her to a therapist. If it leads to conflicts with your husband, then try to stay out of their relationship. Although, it's weird to me that he's okay with it. But the best way to bring up taste from childhood, I try to bring up my daughter that way. Since childhood I buy her stylish clothes, for example, in a children's clothing store kamelino.ro regularly buy something new for the season. But perhaps in your situation it`s easier for you not to pay attention to it.

I agree above. SD11 doesn't dress "scantily", but dresses in all men's clothing. Jean, t-shirts, sweatshirts, hawaiian shirts, etc. It's not the men's style that "bothers" me. But she is just unhygenic. No deodorant, no brushing hair, no bras... Neither DH or BM enforce these things, so I can't care more than them. It's embarassing. 

Dress standards have changed a LOT since we were all teenagers and showing flesh is the norm now. I have a friend with three teenage/adult girls (17, 19 and 21) and these girls are amazing, intelligent, absolutely lovely and even those girls dress in things that shock me ie dresses and tops with huge amounts of flesh showing etc. 

I'm not justifying it but I do think if we are comparing how teenagers dress now to how they did in our day then it's comparing apples and oranges. My SD16 isn't too bad but certainly likes to show some flesh which we don't like. I also wish that her taste was for classy clothes but it's mostly tacky stuff that she loves - her formal dress choices this year (she's had 4 formals) were all in my view tacky but it's her choice in the end and BM seems to have no issue with it.

I would advise to choose your battles. If it's really, really bad then it's your DH job to say something (not you) but otherwise I think overlooking a certain amount of tracky clothing is part and parcel of raising teenagers.