I would consider it experimental if both children were willing participants. If the young sister was forced to do this, it is sexual abuse. The younger child's boundaries weren't respected and she was violated.

I also feel strongly that the older child (by age 12) knew this was wrong. I know that young children acting out sex is often tied to being a victim themselves because small children shouldn't even know what sex is, but age 12 is pretty mature. I knew what sex was at 12. A lot of sexual experimentation begins around puberty, but at that point you also know not to force or bully someone to into sexual situations and you know what isn't appropriate to do with a sibling.