My former SD had been dressing like that since 12. She even left the house and refused to come over because I had a rule of no nipples or vaginas showing in front of her stepbrother. It's been 4 years she still dresses like that even to a funeral no less and her father has taught himself to be oblivious.

My bioparents are together,my biodad is kind of a wimp. When I tried to dress sexy when I was a teen (which is pretty normal to try) he wouldn't sta anything to me but he'd ask my mom to talk to me. 

A divorced dad wouldn't dream of asking his ex to have a talk, it would turn it would turn into a fight, especially if the daughter dresses like his ex. And he won't ask the stepmom to say anything either because that would be admitted there's something wrong with his child. 

They notice, and they put themselves in denial. And if anyone says anything they will attack and defend the kid and deny she's dressed inappropriately.

I grew up in an intact family, and my father had NO problem telling me if he didn't like the way I was dressing.  I had nothing approaching double Ds, but what I did have was expected to be fully covered and I was not allowed to "dress like a streetwalker," as my dad put it, whether I was at home or going out.  My skids are still pretty young (the oldest is approaching 10), but I anticipate no problems because BM is stricter than DH and I are with respect to the SDs' clothes (for instance, we'd allow a crop top while BM won't).  In any event, you're not prudish - there's no need for a young girl to leave nothing to the imagination.

if I attempted to leave the house in what was 1970s inappropriate, my happy ass wouldn't have made it near the front door before being sent back to cover up so I could be grounded. 

I waffle on this. On one hand, she's technically an adult and can dress how she wants. On the other, she's living in her parent's house, and if they have rules, that's what you follow. On my third hand, I'm not a fan of slut-shaming women for what they wear. On my fourth hand, if how she dresses makes others in the house uncomfortable (just like if her brother were walking around in just boxers), it's polite to wear "reasonable clothes" in common areas.

I honestly could keep growing arms and hands to go back and forth on this. I think where I ultimately land is that nudity isn't something to be ashamed about and she shouldn't be shamed for choosing to dress less modestly than you would. She is an adult and has body autonomy. What she DOESN'T have, though, is financial and housing autonomy. That means she has to abide by the rules of the house. If she doesn't like that, then she is free to move out and live elsewhere.

I think the best compromise here is for everyone to be covered in at least shorts and a full-length shirt when lounging around in common areas. If SD is going to be tanning/swimming/working outside, bits should be covered. She can do as she wishes in her room and when she goes out. 

And yes, her dad should talk to her about this. All because they're family doesn't mean they all want to see each other's nether regions. Then again, if everyone is comfortable with being semi-naked and nudity is an issue, your DH may not be on your side with this. That will be a different conversation you'll have to have with him.

and both my parents would of said something to me. My dad growing up always speaks up if he thought something was too short or whatever. Heck, even now as a haha funny even if there is nothing wrong with what I am wearing he likes to tease me and say you are wearing that? My father is a very out spoken person though and got a kick out of annoying me as a teenager. I think step or bio, either adult in the household can say something to the children living in their house and how they are dressing. I am a woman and wouldn't want my SD flashing any of her privates at me while I am trying to lounge in my living room.