A 15 year old walking around the house naked in front of his mother and sister? Bad form! A pre-puberlesent boy or girl prancing around the house naked seems (almost) innocent and therefore excusable enough, but after puberty, the idea should be a non-starter. I don't see it doing any good anyway. His problem isn't being naked around other people. It's seeing other naked guys around him. He has to get used to being around other naked guys. Mom and sis can't help here. They are not going to walk around the house naked whenever he is home, and they can't be guys. The only solution that I see is to continue to use the showers and get erections until the novelty wears off and he doesn't get erections any more. That shouldn't take all that long. Until then, dirty looks may be uncomfortable for him, but in this day and age, nobody is going to try to have him arrested, scream at him, beat him up or kill him. By his tenth "shower with the boys" session, he should be saying, "I'm glad I don't do that anymore!"
Your younger and the excitement of being naked and the warm water is going to get a response. Any one the gives you dirty looks needs to grow up. Unless you are beating off don't worry about it. Most guys may notice and some smile mainly because they know you are younger and at that age you bone up more often. Continue taking your shower and do cover up with your towel when you are finished.. You will grow out of it the more you get use to being around naked men while you are in group showers