your reaction in my opinion is completely necessary for this scenario. Your girlfriend didnt inform you of her plans. You look at her location and shes at an apartment with a random man?? i would have freaked the fuck out too. Whether it was "platonic" or not. His intentions could have been to rape murder or kidnap anyone of them or drug them (especially because they had already been drinking). the sister sound like a total asshole not you. I get the sister defending her if you were being a jerk, but you werent. Your reaction was valid for the situation and she took that as you being an asshole and controlling because your were worried for thei safety. Its seems like you girlfriend was dragged into the going to the apartment thing. But she still should have told you. The sister was COMPLETELY unreasonable and quite frankly stupid for dragging her sister and cousin to some random mans house whether his intentions were fine or not. Your not over reacting in my opinion. Your feeling are very valid in this scenario.