Is it normal for siblings to bad touch each other when theyre little ?

Okay so i was watching Law and Order: svu when i remembered this and this is really awkward and embarrassing but its been bothering me since then. My sister and I are 4 years apart and when we were little kids one time we got a little curious like children normally do. I saw something on tv i think and it lead to us trying to figure out what all the fuss was about.we took off our night gowms. We didnt know what vaginas were at the time so we called it tacos. We basically poked each others tacos. Niether of us felt anything, we tried rubbing tummies which is what it looked like on screen, and again nothing but i remember her saying her tummy hurt(we both took turns laying on top of each other and rubbing tummies) the next thing which is what i now know to be an attempt at oral, but nothing like what me and my boyfriend do. She just licked my taco one time(literally one lick) and it was really gross we both said eww afterwards and agreed never to talk about it again... i remember saying mom would be mad at us. But anyways since i am the oldest i feel extremely freaked out and guilty about it...like gross incest,and i was the oldest so its my fault for not knowing better or for even doing that with my little sister. I dont want to bring it up and freak my sister out. I mean yea it was only the one time but it makes my skin crawl just thinking about it. Is this normal or do i need a therapist???