Confession?

So, I am 25 years old & have NEVER told anyone about this. (Remaining anonymous) when I was little (3rd grader - 7 y/o), my sister and I still took baths together (she was only 8 y/o). Well, she had already started her period and had all the hormones. When we were in the bath, she would have me touch her and vice versa, we even did oral. This went on for a while actually, I'm not even sure how long. I didn't know, at the time, that had anything to do with hormones or periods or being "a woman" or whatever. I knew nothing, I was an innocent kid. I never told, I don't know why, maybe she told me not to, or maybe I was scared to. I just don't know if I should ever say anything. But it has been almost 20 years that I've kept this in my mind. It's so weird to me. I'm considering telling my husband and most likely will. This isn't something I purposely kept from him, it's just something I don't think about too often and for some reason it just popped into my head.

So...should I speak up? Should I tell my husband? What about telling my parents, at least my mom?

Edit! If I tell my mom and my sister denies it, my mom will NOT believe me. My mom has always believed my sister over me, throughout our lives.