So make a deal. If she either puts off having sex until it's legally allowed or makes sure she's both sober and protected, you'll keep shtoom. Make sure she knows you mean it and that there must be no more accidents. And you should also encourage her to talk to her own mum. She might be far more understanding than your stepdaughter believes.
Mums usually are.
But your stepdaughter needs to believe you are on her side, so don't betray her confidence. If you go telling tales, she'll never be able to rely on you again. I HAVE two young daughters and if my former husband's partner kept this sort of thing from me, I would hit the roof. In other words, it's not her flesh and blood. She should have refused to interfere and referred the girl to her real mother.