>>87573
In my experience this is mainly an only-child thing. They feel comfortable enough to walk around the house in their underwear without being judged. Usually, siblings are competitive and degrade each other, discouraging them from wearing less around the house.
Boys are different because they have a lot less pressure on them to look physically attractive, it's more common for them to walk around in boxers/briefs and/or a shirt.
OP's stepdaughter is at an age where she is extremely self conscious about her body among peers. When she's home, she can dress however she wants, and walk around in next to nothing when it's hot. She does this because she trusts you and her mother to not take advantage of her or put her down for her looks.
Here's an example:
When I was 12-14 I had a crush on an older man, and was just beginning to explore my sexuality by trying makeup and wearing more revealing clothes. I visited mom on the weekends and her boyfriend visited her every day, so he would walk in to find me watching TV in my mom's short satin nightgown. Looking back on it, he probably thought I was trying to tempt him, but I genuinely just wanted to practice feeling comfortable as a "woman" (and have something to tease my crush about).
OP, it may be uncomfortable to see her half-naked all the time, especially if you're hebe, but trust me, she thinks you're already in love with, and exclusively attracted to, her mother. Pursue her and she will learn to never trust anyone else because she trusts you to remain faithful to her mom.
Also remember the exact plot of Lolita. She may even say outright that she wants you but don't even go there OP; she will grow up eventually, understand, and hate you. If you hit it. she can and will use that as leverage against you if you ever piss her off in any way. And god help you if her mother ever finds out.
Don't listen to the other anons here trying to live vicariously through you or goad you into testing the waters. Be a good stepdad, for her sake and for yours.