Hi Bev,
Maybe in some way your brother is reaching out by inviting you. Yes the instruction to come as male hurts a lot, but you were going to attend in that form. Sometimes our feelings can be a barrier to the longer term gains. This could be a first step to repairing your relationship without your brother feeling anxiety about how 'he' would be perceived having you come as the person you truly are.
In a small way we all compromise our needs, My wife and I were going to a function the other night and I came out to the lounge with my woman ankle boots under my jeans rather than my bloke ones. She very quickly spotted this and told me in no uncertain terms to change. I did this as a tongue in cheek opportunity to see if she does check me out carefully as she is extremely fearful of my hidden life becoming public. I duly changed but hated that I had to present fully in ala-male-mode apart from a camisole.
I know it is a very weak comparison, it's just you have an opportunity to start the recovery of your relationship. Maybe his invitation and not acceptance is his best effort to start the ball rolling.
Only you will know your brothers intentions and if the invitation is sincere enough to travel the distance.
I do hope you find a way to connect as it would be so sad to lose a connection with a sibling.
Hugs
Sue.