Stopped sleep overs at nanas place due to cousin inappropriate touching, need advise.

When my daughter was about a year old I started dropping her off for weekend visits at her grandparents home. My husbands sister and her 2 year old son live there too. At first I didn’t mind that the kids were bathed together since they were so young and it was convenient to do that. As time went on my husbands sister started hinting that the kids were touching each other then I started feeling uneasy and asked for the kids to be bathed and dressed separately. My sister in-law ignored my request, she even went to the extent of telling me cousins learn these things from each other. I felt so devastated when she said that but left it alone for the sake of peace.

When my daughter was 3 years old in the shower with me she started asking about my genitalia, I wasn’t prepared for this but figured I’d rather she learn from me so I encouraged her curiosity. She then asked why my “cookie” was black (I have lost o hair down there), I then went on to explain we grow hair on different parts of our body etc, then she said her cookie is pink because her cousin told her so. I got concerned and started digging to find out how he knew. Long story short, the thing that bugged me the most is that at some point during her visits to grandmas place her cousin told her to make their cookies kiss, when I asked her why she agreed to do it as she knew nobody was allowed to touch her cookie she told me it was because “he said they MUST do it”. My daughter is very impressionable and always tries to please everyone, especially her cousin, they’d get up to mischief and almost always it would be something he made them do. I had tried to teach my daughter about appropriate and inappropriate touching from the time she could talk but I felt so defeated that day. I was sexually abused as a child by my cousins (young and old) and tried so hard not to impose my fears on her but now I don’t know what to do to. After the shower we went straight to her dad and I asked her to tell him what she told me. Because daddy feels like he grew up as her cousin touching his cousins he felt reluctant to bring the topic up with his parents or sister. I eventually demanded that the child does not visit anyone’s house without him or me present as I could see he was starting to feel uneasy about the whole thing. This didn’t sit well with her gran and I tried to explain why we decided to do this without bringing up my past but it just got complicated and we left the issue. Our daughter is now 6 and I feel so lost. I feel like I’m sheltering her instead of equipping her with the right tools to protect herself when I’m not there. Was the decision to ban sleepovers a good or bad idea ? How do you teach an impressionable child that although some people are not strangers and we need to be kind that some things are non negotiable. I understand they are kids and curiosity is normal but how do I stop history from repeating itself 😔. I grew up feeling guilty and dirty for letting certain things happen to me and I do not want my daughter growing up that way. I feel like I have failed her and really need advise on how to move forward. I can’t keep her from sleeping over forever.