Hey Tammy, you absolutely must go in my opinion. We are at a similar level of transition and if you are anything like me, you are likely getting close to the point of total male fail. You have a golden opportunity to have a memorable time with your brother just as the old you (assuming that is what you want to do), the next time around it may be totally out of your hands whether you tell him as it will be obvious to all who see you. That's not to put any pressure on you of course, tell him when you are good and ready and not before would be my advice unless he asks directly.
Try and get past the anxiety and whatever you do don't bail on him, you will end up regretting it I'm sure. Just enjoy your time with him and if the timing feels right, tell him, if not, treasure the memory in case he reacts badly when you do.
My two cents, not saying it's right or anything but that is what I would do.
You've got this.