Was it sexual abuse?

When I was young, around 7 years old me and my younger brother had a girl around 15/16 babysit us when my mum and dad went out. When it was just me and her she used to undress and ask me to touch her breasts and she would do the same to me. I don't know if she ever did the same with my little brother and I would be too embarrassed to ask him. I kinda forgot about it when we moved and she stopped sitting us also at the time I don't remember thinking she was doing much wrong. She never hurt me really. Over the last few years I have thought about it more and one night last year I got drunk and told my boyfriend (first person I've told) I don't know whether or not this is actually sexual abuse. I also wonder why she would have wanted to do that with a young kid at 16 years old, maybe this was happening to her at home. I dunno but I can't stop thinking about it lately I don't know whether or not I should tell someone else