Dating your step sister is weird... right?

This is a little bit long but like... I have to tell somebody. Okay so... I went to middle school with this kid (we’ll call him F) and up until around 6th grade we were really good friends. We slowly drifted apart but we still kept each other in social media and all that good stuff. So I see him posting up this girl (we’ll call her H), buying her stuff, and even posting sexual snaps of him grabbing her ass and her grinding on him and stuff like that. So obviously... I assume like any normal person that it’s his girlfriend. But one day, I see her instagram. She had a post of her dad, saying happy Father’s Day. But I know I’ve seen this man before, he’s F’s moms husband. I almost died in that moment, so I went on some detective Sherlock Holmes shit to make sure I wasn’t wrong about this. At this point I’m 99% sure I’m not, but I HAVE to ask. I ask, immediately he’s defensive and mad. He also sees nothing wrong with this, and I know it’s not technically illegal but like... still. But I end up getting threatened to be beat up, and ultimately blocked. But that’s okay because I figure I’ll never see these people again in my life, well I was wrong. I was at lunch during school today and guess who walks right by me? H. I wanted to post this all on Instagram but F’s mom still follows me, and I’m pretty positive his parents don’t know they’re dating.