I understand why you are concerned. It’s not usual for an 11 year old to strip just because a younger one did so. However, there may be extenuating circumstances. Kids mature at different rates. Families have different ideas about nudity. Sometimes kids aren’t “bad” but are showing bad judgment. I can’t comment on what the psychologist said about normalcy because I don’t have access to the same information she did.
The family did many things right in this situation. An investigation took place. The little boy was checked medically. D’s mother had a long talk with him and has taken him for counseling. You’ve done your best to continue to be in relationship to the family.
I think the best thing to do now is to talk carefully with your 6-year-old about what is and isn’t okay regarding nudity with anyone but her parents and doctors. Many parents find that it is helpful to tell their child that no one should see or touch any part of the body that is covered by a swimsuit. Also help her understand that she should come to you if anyone does such a thing — no matter what. Even if someone threatens her or tells her not to tell or else, she needs to come to you.
The 11-year-old is actually your daughter’s uncle. Another possibility is for his mother to talk to him about his role as “uncle” is to help to protect his niece. Being given responsibility may help him act more maturely.
Family relationships are often challenging. The tricky thing in this kind of situation is that it’s important to keep two things in mind: You want to protect your daughter, of course. It’s also important not to let a child (D) get labelled as an offender when he may have once made a bad choice.
If I were in this situation, I would try my best to normalize the relationship with the mom and her son but I would also be very vigilant for inappropriate behavior. Then take it from there.
I wish you well.