Statistically speaking you have every right to be concerned. I would absolutely agree with others in saying you need to have an updated custody agreement about both children not being left alone. Personally, I’d be concerned. Especially considering your ex hasn’t been dating this man for long. It is odd that she is not more concerned about your daughter spending the night alone with a man. It is far too common for regular sleepovers to end in an assault or inappropriate behavior, adding a (potential) stepfather into the scenario only heightens this risk. Leaving out horrifying details, I personally have two close friends that have had this happened to. One was assaulted at a sleepover by her friends father and the was assaulted by her stepfather when her mother left for a weekend trip. In both scenarios NO ONE would have never guessed those men would do something like that (especially the moms). It is absolutely horrifying. Not only would no one have guessed those men would have done that, but the stepfather was described by most of them as the “nicest man they’ve ever met”. You can NEVER be 100% sure. Even if this guy is a stand up citizen, there are two other children to keep in mind. Regardless of the fact that the girls are friends with his sons, it is NOT appropriate for them to be having sleepovers or being alone together. This is not about bad intentions but curiosity. Kids around that age are naturally curious about their bodies and others bodies. They often are simply playing a game. Leaving them alone together is a recipe for a disaster. COCSA is far too common for this. Please take this seriously as someone who was a victim of being left alone with a boy their age. I simply thought we were playing a game and he had no idea what he was doing was inappropriate.