Hi @Mbar15 ,
Welcome to the parents forums, I hope that you find this community helpful 
Dealing with teenagers becoming sexually active is a pretty challenging time for parents and carers, and I'm sure a lot of parents here will have been through it and will hopefully have some helpful advice for you. There's also a lot of resources on the ReachOut site, with suggestions about how to talk to teens about sex, safety, consent, and all of the tricky conversations around sex and sexuality here https://parents.au.reachout.com/common-concerns/everyday-issues/sex-and-teenagers
How old is your stepdaughter? It must be hard hearing these sorts of stories second hand, do you feel comfortable talking to her about what you've heard?
You sound like you really care about her a lot, having strong relationships with teenagers is so important and you've done a great thing reaching out here 
