Is it illegal?

I’m worried that I might have broken the law. So I’ll just get it off my chest because I don’t want to tell anyone about it but really need to know. I have an older sister who is legal and I’m 15 and we have sex together a lot. One time I told her that I wanted to experiment a little so she said no but then I asked for it the next day and she said I was too young and didn’t know what I was doing. I said I was gonna do it anyway. So we did it and it was the most painful thing I’ve ever done but it felt really good. Now I’m worried that because I’m under the age of 18 and she let me do it that she might be breaking the law.