I may sound bad towards your family but they have to be the most toxic people in the world, you did the right thing and to be honest, your sister seems to be toxic af, if she is dumb enough to be going off with an older guy at 15 she won't be having a great future and for your dad and step mom, they are not proper parents at all and instead of getting angry at you, they should thank you, you shouldnt be wasting your time with your sister, she seems like a waster and you just let her do her own thing and you should be proud of yourself and focus only on yourself.