It's not shaming. It is sexualization. The step father feels uncomfortable seeing the 15 year old wearing a towel. Yes, that means she looks sexy and he is disturbed to think of his step daughter as arousing. Also, the 13 year old step brother, new to this girl in his life, must be thinking the same.

True, if they were at the beach, she'd be wearing a bikini, but that's what that is.

Why do we put something on when a stranger comes to the door? Do women answer the door wearing a towel?

It's not shaming the girl for her mother to teach her to wear appropriate clothing. Everybody has their own line about this. Some families are nudists, some are Puritanical. Most are in between.

I remember the age I was aware of not being too naked around males was approx. 7. Geez, maybe even younger. My mother wrote in my baby book how at 2, she asked me to go get something outside, and I said I couldn't because I was only in my underwear and there were boys out there. So, I picked up on that myself and thought that by myself from that young.

Is it shaming for a young woman to think 'I should not walk around so undressed my step father and brother are uncomfortable because they feel it is inappropriate (that I am evoking sexual thoughts from them because I am wearing a towel and have a sexy body)'. I don't see that as shame. I see it as just there's a time and a place for everything.

I'm not shaming the males in this either. The OP could act like it doesn't effect him, meanwhile being aroused by his young step daughter. At least he is honorable enough to ask her not to do it and tell them it is bothering him. Of course it is due to her sexuality. Why else would he care that she is in a towel?