Is it normal that my girlfriend wants to involve her sister?

My girlfriend is always wanting sex which is no problem but lately she started telling me her fantasies and we took care of most of them but in one of them she wants to involver her sister they are both almost unnaturally sexy and willing. I have no problem with it but I think that it might end up changing our relationship because her sister has told me before that she wants to be with me as my girlfriend maybe for better maybe for worse. Ive heard them talking about both being with me sometimes they just mean sex sometimes they mean more. I was just wondering what everyone thinks I should do and what they think of the whole idea. SORRY FOR POSTING SUCH A LONG QUESTION GUYS I JUST LIKE TO BE THOROUGH.