Not the bad porn script you were expecting, I swear. 🙃

So here's one for fans of grey areas.

SO has brought two daughters and a son into our relationship. They're all young adults now. We generally get along well and I'm a trusted third party and, according to the then-minor younger daughter, a good source for pregnancy tests and the such "because you're the least likely to freak out".

We're generally a very open and judgement-free family, and I'm immensely proud of the fact that the kids feel comfortable talking freely about love and sex with both of us despite their 'traditional' catholic ~~indoctrination~~ upbringing. We've established that we can talk about anything but won't go into detail about their or my personal preferences. This works very well and there's a lot of trust. Whenever their mother's not around, the kids talk to me about whatever's on their mind (anything really, not just love/sex stuff).

There's one thing that makes me slightly uncomfortable though, and I'm not sure if I'm the one who's at fault here. It’s the older daughter’s choice of clothing at home. Especially in summer and near the pool, she often walks around in a t-shirt or scant bikini top and panties whose front just about covers the crack and whose back leaves nothing to the imagination.

I'm very happy that she's both happy enough with her body and feels comfortable enough around me to walk around this way. Especially the former has been a bit of work on her mother's part.

Now my own background is that I come from a very uptight family myself (sex is an evil and shameful thing that the wife endures because she owes it to her husband) but am very open now (swinger clubs, former co-host for BDSM meetings etc.). I also have a minor degree in both communication and sex therapy.
Still you never entirely shake off an upbringing based entirely on shame and guilt, and occasionally there’s a situation that instinctively makes me uncomfortable, but upon closer inspection I conclude that it really shouldn’t.

I’m currently trying to figure out whether this is one of those false-alarm situations, and it really bothers me.
On the one hand, "you don’t wear that kind of outfit in front of men you’re not biologically related to."
On the other hand, why not? She should be able to wear whatever she’s comfortable with in the privacy of our home. And you can't make a request such as "dress more modestly" without it smelling of misogyny.
On the other other hand, it would definitely be inappropriate for her to walk around entirely in the nude, so there's got to be a line somewhere.

(Let me add that she’s never even remotely tried anything, and I really don’t think of her in 'that' way, so that’s not the issue.)

TL;DR: Adult stepdaughter sometimes wears revealing clothes, makes me uncomfortable, not sure if it’s supposed to. Nothing fishy going on.

EDIT: Wow, so much food for thought. I don't think I'm going to be able to reply to everyone individually, but I've come to realize that what makes me uncomfortable is probably the idea that she or other people around us might feel uncomfortable, and in the classic stepfather-stepdaughter constellation I could be seen as a potential 'perpetrator', which I'd need to protect myself against. So it's really just mostly me worrying about what other people might think (but probably don't).
But the point is, if she's comfortable there's really nothing tangible for me to worry about.