Child-on-child sexual abuse is definitely a thing, and this sounds like an incident of it. If you want to seek more specialized support, consider looking for groups for survivors of child-on-child sexual abuse. It's a very difficult type of trauma to process.
You are not over reacting. It was wrong of your parents to not tell you ANYTHING. It was wrong of your cousin to get you to do things with him. At age 11 you are not old enough to say yes or no and that is a completely different subject than whether or not you were physical hurt or if you enjoyed it. At age 12 your cousin is on the borderline, in one way he should have known it wrong to be involved that way with a younger relative but on the other hand he may not have been old enough to understand himself what he was doing wrong. My dad molested and sexually abused many children and one time even encouraged me to touch my friend. Since I was only 12-13 I didn't know what I was doing was abusing her and to this day pray she forgives me. Your cousin could have blocked the memory of the experience himself and have no idea why you avoid him. He could remember and be ashamed and want to apologize or he may remember how easy it was to convince you and try to again. Only you and he will know which one. Be careful, get help, it's good you reached out here!
Yes, you were abused by your cousin. How old was he at the time? Child on child sexual abuse happens and is usually, unfortunately, part of a cycle. Your cousin was likely abused by someone else and then became the abuser. You have every right to be upset that he stole a piece of you with this. But don't let it define you. I think avoiding him is key and if he is in a position to abuse ANYONE else, I think you may need to act to stop that. A therapist to work through your own emotions of it may be helpful. Sorry this happened to you!!!