Inappropriate Conversations?

I’m not sure if this is the right place for this but I don’t really have anyone else to ask.

I have a 21 year old little sister who is living her life and exploring her sexuality as 21 year olds do. We have a good bond and she trusts me and wants to talk about sex and relationship issues with me, which is fine. The issue is that she is often babysitting our 14 year old autistic brother when she calls me to talk about this stuff and I don’t know he’s there listening until he speaks up. His mom (my stepmom) doesn’t think he’s ready to hear this kind of talk, but my sister thinks it’s important for him to be educated and have that knowledge because he’s getting to be a teenager and no one else is having that talk with him because of his autism.

I’m caught in a tough spot because I don’t want to disrespect my stepmom, I also don’t necessarily want the pressure of giving sex education to my siblings, but she has a point. He’s 14 and has already expressed certain feelings and desires and I want him to be safe. Still, I don’t think he’s necessarily ready to hear all the things my sister feels so comfortable over sharing in front of him. Am I being a prude? How do I approach helping either of them without crossing boundaries? Should I tell my stepmom and have her handle the lines my sister may be crossing even though I may be the only one willing to listen/talk to them about uncomfortable topics?