She continues to ask my girlfriend to go to these parties.

sister’s always stick together, I know this from personal experiance… what I mean is that I have to older sister’s and were all three years apart from one another…before guys came into our lives we did everything together, we were like the bestest of friends. But after guys entered our life we split apart, and barely had the chance to talk to one another…guys hate it when girls talk cause it’s like they assume they’re talking about the guys…and the guys we were with hated it we just wanted girl time… so after years of noticing the fact that guys were trying to split us up we had to start lying to them, and become rebelious (it shouldnt have to come to this since we were family and they just came along) guys need to stop acting like they know whats best for us, only we can make thoose decisions…if you can’t handle the fact that they’re doing things together and without you then maybe you should find someone who doesn’t like to have fun? sorry if this sounds rude, I dont mean it towards you…I just cant stand the thought of guys trying to tell girls what’s best for them… but if you do want to show her how it feels I guess you could tell her your going to a dirty halloween costume party…or going clubbing with some guys and see how she reacts…after she reacts then you can tell her you were just joking (or actually make plans and do it)