It is normal for you to be jealous of your girlfriend. If your girlfriend and friends, no matter who they are, are playing well, shoulder to shoulder, singing K or taking selfies, it is not normal. It's nothing, because she is cheerful and lively, and many people can make friends, but if she is like her brother, if they grew up together. Good. There's nothing here. Even my cousin is related by blood and has this affection. Although mine is my own brother, I can understand that feeling. That kind of affection is very rare, so I will cherish your girlfriend very much, and it will be no exception.