Am I overreacting?

We used to have a pool in our backyard. One day my 5 yo daughter and 4 yo niece were out back swimming, (it’s like 3’ deep, they were in life jackets, and I was watching them intermittently from the window) and my 10yo nephew came over. He hopped into the pool with the girlies and all was well and good. Until I looked out of the window and he was too close to my niece for my comfort. I walked outside and he immediately swam to the other side of the pool. My 5 yo innocently said “Mom! Were playing truth or dare and (nephew) dared us to kiss him on the mouth!” I (knee jerk) said “excuse me?!” He just looked at me and looked away. I made them all get out and come inside, and I told all parents. Thing is- everyone blew it off like it was no biggie. I don’t feel comfortable leaving my (3) daughters alone with he, his dad (who has always given me the creeps) or his brother (9 yo). Am I overreacting? I’m going with my gut and not going to change course completely, but do I maybe need to check myself? via /r/Parenting https://ift.tt/2LKF54o