You need to tell her dad and let him deal with it.

WOW!! I would just talk to your BD. Let her know that having sex at her age is NOT ok. Make sure to have the talk with her if you haven't already. You have no control over what SD says, but make sure to let your BD know right from wrong. I would be absolutely freaked but you just gotta keep tabs on your girl. It's great she talks to you.

I think I would do everything in my power to keep BD and SD away from each other. She could lead your kid down a bad road.

SCARY! We have SD here who at the tender age of 11 was smoking, sneaking out to boys and at the tender age of 13 had three STD's and then went on to use drugs and alcohol. She's now in a secure facility and we can sleep at nite!
I am aware that any comments or advice I can give are in retrospect. There is only one girl here, already damaged so badly before I came into her life. I am still not sure how much I have changed anything really. I did get from her that she did some thinking and now realizes that the things I did were actually for her, I was so delighted to see a glimmer of introspection in her.

I think the best thing is endeavor to instil into BD that she may listen to SD but urge her not to do the things suggested because it will likely end badly for her.

I shared the movie 'girl interrupted' with SD but, unfortunately, she liked the Angelina Jolie character more! Hmmm...

I think sharing different things with each of them but still finding some common ground.
Being a 'big sister' can be a burden on an older child too...an immature 14 year old imposing her ideas on sex is probably her way of 'educating' her?
Just throwing ideas out here...

Um, yeah. This is my fear. Not that my ss13 would be having sex or encouraging my kids to as I feel he would have to search quite a bit to find a willing partner for his 8 year old pee ridden body (yes, that is harsh, sorry)But that he would be talking to my kids about any inappropriate things-drugs, alcohol, delinquency, etc. Fortuntately in my case my kids dislike my ss so much they think pretty much everything he says is ignorant. However, he likes to tell stories to my youngest who is only 9. He bragged to her last year about stealing my bottle of alcohol and drinking 1/2 of it. She was 8. I am still smarting over that one as because like so many other things it was not adequately (or perhaps more accurately not at all)addressed by my dh. But you can bet your boots I addressed it with my dd.

I would speak frankly with any of my children about the dangers of sexual promiscousity, stds, pregnancy-I would also tell my childrn that my skid had problems and that is why they talk about/do such things and that is not a normal, appropriate thing to be doing at that age. I would also go to my skid and quite frankly threaten them within an inch of their life to keep their mouth shut and stay away from my kid-then I would try to force my dh to ground her to her room for the next 4 years until she moves out since she is sexually promiscous. And yes, I would find lots of activites for my dd to do that were far away from the wicked stepsister.

Chances are if you have a guilty biodad and tell him about it, he'll take offense and ignore it anyway.

Protect your BD at all costs. Have a chat with her about your personal ethics and views and how it's inappropriate for SD to be talking like this to her.

Have her respond to any question that SD asks with "I'll get back to you on that" so that she doesn't open herself up to an onslaught.