Your step-brother was probably sexually abused and he was doing with you what he learned how to do when he was abused. At that young of an age (of the abuser), it's not legally labeled "abuse" but I think it still has some of the same effects. This would be a tough thing to grapple with for sure. I think the way to make sense of it would be to understand you were both so young, and both impressionable. Both exposed to things you shouldn't have been exposed to, at a very young and innocent age.

It is not uncommon for young kids who have been abused to go on to do the same thing to other kids, unfortunately. Not unlike how kids who get hit are more likely to hit other kids.

I hope this helps.
Angela