I would'nt get too concerned about this, as others have said it's fairly normal since most young people are naturally very curious about the opposite sex and siblings offer a 'safe' way to find out about the equipement.

Sounds pretty normal. When you as young as you are you probably have very little experience with the opposite sex. If I'm wrong in this case, it isn't personal. Any way, the titillation that accompanies having anyone see your cock out isn't exactly linked to who it is. It's more the un-normal situation that is the turn on. I had a cousin that I could never fuck but as teenagers, younger teenagers mind you, we spent a lot of time checking out each others growing bodies. She showed me her pussy and boobs and I showed her my penis. Once, she asked me to make it hard and I did. We only looked and didn't touch but it was very erotic. My guess is that this will pass as you grow older and get used to being naked in front of women. Sister or not, it sounds like a natural phase to me. At least that's my opinion given your statement.