What
@Dark.Green.Feathers wrote is really excellent advice
@ladybug111 ... from the outside looking in, it's easy for us to see your situation as just a usual normal interaction that happens between lots of kids.. completely non abusive due to the ages you were and the fact that you were both doing it (doesn't seem like anyone was forced to do anything)... but i also know that 100 people on here could answer you in this way, and it wouldn't change much about how you feel about it... which is why maybe talking with a therapist is about the factors which are making you feel so awful about this situation could be important... because it seems all of this is effecting and distorting your view of yourself.... and therein lies the damage..
In COCSA the key thing for the person being violated is whether they felt there was a power differential... that they had no control over... and were therefore forced to do things they didn't want to do or which hurt them... in the situation you describe above, it seems like your sister was fairly equal to you in the dynamic in that you both participated. Yes you were older, but you were only 10 yourself and age on it's own is not a power differential. Just a biological fact. It has to go hand in hand with superior knowledge for example. But from what you've written you guys were exploring what it meant to have female body parts. You thought back then it was harmless - and this is key. If you'd said you knew it was harmful but for some reason you continued there would be something messier to explore. But your intention at this age was too explore without harm.
So what if we really analyse what you've said too drill down a bit? As someone who experienced significant cocsa growing up, i read what you've written and can pull out some themes which are missing from your experience which, to me, doesn't make your experience cocsa, or you am abuser.
My personal opinion on it all (from the info you've shared) iis that you weren't trying to trick or coerce your sister in any way (physically or emotionally). You didn't try to persuade her into doing something by telling lies or pulling the wool over her eyes. You didn't guilt trip her. You didn't appear to want to use her for your own sexual gain and do whatever you needed to do in order for that to happen.
I'm not sure if any of that helps at all. I understand you still may feel negative about yourself and if you do I would explore talking in therapy about this. Because the damage for you will be the hiding of this, the secrecy, the holding of it on your own and not getting perspective which can lead to your sense of self and identity being shaped incorrectly and negatively. So do take this seriously to help yourself.... you haven't done anything wrong