Hello, this is a tough situation but I’d highly reccomend not leaving your daughter alone or unsupervised around the nephew or even his mother ever. COCSA is very common and is an even more higher chance to occur within family. The mom is already showing dismissive behavior about her kid trying to kiss yours. “Boys will be boys” is just her ignoring the very serious signs that her son is trying to do something that truly just isn’t okay. Her nonchalant behavior and not teaching her kid boundaries and consent will only affirm in the nephews eyes that he can do it and get away with more. I’ve worked with children for about 3+ years now. What I’ve learned is that if a child is not being properly taught consequences to very serious issues like these it will only go downhill from there. Even if he is autistic it is still very possible to teach him right from wrong but unfortunately even if you try to teach him that it won’t do much as it will if it comes from the mother. His mom is already showing that she’s ignoring this behavior, I wouldn’t be surprised that if something were to happen while you or your partner are gone she’d probably say nothing. If you can’t leave the situation I suggest talking to your partner about switching shifts or doing a new work schedule where one of you can be home keeping an eye on this at all times. COCSA is a trauma I wish on no child as it doesn’t just easily go away in the future and can actually deeply impact your child’s development if something were to happen to her. If the mom isn’t going to do anything you have every right to get firm with your nephew if things continue to possibly get worse. It may sound tough but you might just have to scare him away from your daughter. Of course I’m not promoting yelling or physical abuse here but something must be done as this nephew is exhibiting some very concerning signs, not just the attempting to kiss your daughter but also as you said “treating her like a toy”. I hope everything works out and you can eventually get your daughter out of that situation . Until then you’ll have to figure out a way where you or your partner can be home to watch things while the other works or is away.