I want my girlfriend and her sister to myself.
Why do you need to have control over your gf's sister too?
Do you have any problem with that?
You already have a gf so it's immoral that you want her sister too. it's not right. Your gf will leave you for this kind of weird desire.
I'm going to have the girls here and hook up with the both of them.
That is wrong for you to do. Her sister may like someone else and there is bound to be jealousy. It's inappropriate to want to be with your girlfriend's sister.