Abuse exposed

I come from a very large family. I have more cousins than I can count. We always had a house full of family and friends when we were growing up. When I was 3 my Aunt and her kids were staying with us. She had 4 kids at the time the oldest was a 16 year old boy. One night my mom walked into the room and caught my 16 year old cousin molesting me. She kicked them out and never told my dad or the police. I still remember this and it still hurts like it was yesterday. From that day on I did my best to always avoid him at family gatherings. But, deep inside me I blame my mom for not protecting me over him. She let him get away with it. How do you let him get away with doing that to your 3 year old child?