I masturbated to pictures of my sister during early puberty
How could you masturbate to pictures of your sister?
I didn't know she was my sister at that time.
You can masturbate when you want to, but doing it to a family member is wrong. Incest is not something that should be practiced.
What if you didn't know they were related to you?
Incest by definition is a family member related to you. So even if you don't know, it is still incest. It is better to avoid it entirely before it changes how other people look at you.