So normal! And not sexual! How can it be when you don't even know what sex is at that age? Do try to find some tools to help you through your guilty feels though! Guilt can have a lot more negative impact on health then masterbation (which is healthy and normal)
I remember a time when I was around 8, going to daycare. There was another little girl around my age. We used to play house, acting like the mom and dad. We would go into the treehouse and bring the ladder up so the "little kids" wouldn't bother us. One of us would get on the other and we would rub against each other because it felt good...we had no idea it was sexual, just knew moms and dads did that. I know a lot of kids that played around like that, I think it's pretty normal. When I got older and learned about sex and why the rubbing felt good, I felt ashamed..like I had somehow abused the other girl. As an adult, I realized those feeling come from me being an adult, looking back on it with an adults prospective. Not the child prospective. I think it's natural and a lot of kids do it. Don't let it dictate your life, because you are looking at this from the adult prospective.
I had a very similar childhood experience. Not sure how I figured it out, or really even how old I was (but young, under 5 I think). But the feeling would come over me at any time, and I would have to masterbate. My parents were horrified, and told me I would have to have an operation from doing it.
Scarred me for life about sex. I was always ashamed, always felt guilty. Through my entire childhood, I always felt I was a terrible person.
That was over 50 years ago, so am glad people are more open, and not so judgmental.
You didn't abuse your sister - what you were doing was the kind of harmless sensation-play (the term sex-play isn't appropriate) that kids have always engaged in.
And there's no possible link between masturbation and obesity - so chill out!