Goodbye is not easy.

This could prove to be a period of adjustment for you. It doesn't mean the end of a good friendship necessarily. Life is about change friend. Change happens and people need to move on to different things. Sometimes to better things but it shouldn't make you so down. Your friend needed to do what she needed to do at this particular moment in time. You should send your friend the best wishes because they could also have some fear about the changes. A separation at this time could be what you need anyway just to get your mind straight. I am sure that you will have some moments where you will greatly miss this person. You could find a way back to each other in the future under different circumstances. Things have a way of happening in life that could surprise you. Take a look at this circumstance as a chance for new beginnings for yourself. It could be that she is not choosing her brother over you. It is just the choice of the best situation for her at the time. I sure hope some of my words have helped you.