Am I a terrible sister?

To answer the title no you are not a terrible sister. I get you love your brother and want to be there for him but now you have your own life. But you should try to just have some fun with him not protect him. Brothers and sisters best relationship is we can be mean to each other and still love each other. Your feeling this way because all those years of being there for him and you've finally gotten use to not being with him I'm uming, and he's coming back to you right after that time. Just loosen up and treat him like a friend not a dear brother.