Honestly, I might be the minority here but I think telling your mom is a harmful idea. I would leave it or confront them alone. If you choose to confront them, do it all together (both) and ensure that you won't tell anyone what you saw. They need to trust you if you are going to do anything to actually help them and work through this. They have to seriously confront what they are doing and telling you in trust/confidence is a huge first step. Telling your mom will only distance them further from everyone but each other. If they begin to express shame, guilt, fear, etc. be cognizant to it and reinforce humility and understanding (only do confront them if you can do this from a place of love and compassion). *This isn't to justify or rationalize anything, this is solely to help them get it all out with you. Just let them say anything and everything they need to say. After that, you need to calmly explain to them that they are normal as everyone but continuing this unhealthy behavior is destructive. Don't high horse it, just explain the importance behind intimacy and finding a partner with value etc. This is ugly, I'm gonna be honest but you can help them move past it without trauma with some tact. Understand the consequence of what you choose to do. Leave it alone or think about whats best to help them. An older sibling has capacity no one else can give to them.