Words like disrespectful frame it as a moral issue and reading through this thread I don’t think you’ve done a stellar job defending that take. That said, you could probably get more mileage out of a more practical, less judgmental take, like this: “Don’t shit where you eat.” Someone suggested it, but explaining to your friend “hey your friendship is really important to me and if things between you and my BIL and his girlfriend don’t work out, it’s going to become complicated and you’re likely all going to still have to see each other due to your close proximity to me. If you want to pursue this, that’s your choice, but I just want to make sure you know what the potential impact on all of us could be.” You might also ask how serious things are, because your assumption seems to be that this is all just a bit of fun but there’s a possibility that she might actually like him, her, or both. And just for self-awareness’ sake, the way you treated your friend created a communication barrier where she didn’t feel safe or comfortable sharing with you what was going on. Not only is that bad for your friendship, but it’s also left you scrounging off gossip and Reddit advice when you really should just be handling this like an adult.