My sister is out of control?

my sister, mel, just this month turned 15. he is a freshman in HS, and is enjoying it. for the past few months she has ben dating a boy 1 1/2 yr. older than her. she began sneaking out, i told my mom and she talked to her but aparently it hasnt done her any good. now he sneaks into her room through the window at about 3am quite often. and the day before thanksgiving my mom got home early from work, heard mel's radio playing in her room, so she went in. Mel was on the floor with my mom's boyfriend's son, bean (15yrs) ( they all live together) on top of her making out!!!!! he is about to be her step brother!!! other things have happened as well. my mom doesnt know what to do. how can i stop her before she makes a huge mistake??