OP, I'm not even gonna try to advise you about the business side of your dilemma, as I know zero about that subject, well certainly not enough to help you more Han the other educated posters.
I think your post is more about the disappointment you feel, after creating your business, putting in hours on end of hard work, making it as clear as you could that this business of yours is your "baby", and you've trusted a few close people to share.
Now you have discovered that your mum has discussed your business with your db, it no longer feels as special as it did.
You love your mum, and your mum loves you and your brother equally, and so this situation has arisen, it seems, from her with to treat both her children equally.
It's particularly galling (I imagine) that you perceive your db, not even to be all that interested, it's his girlfriend of nine months that seems to have the enthusiasm, and bit between her teeth.
Whether you can do anything about it, I'm at be possible, but then, I think that is upsetting you, as it will show that you no longer trust your mum, and it may well cause a rift that you don't want either.
I don't think there is an answer that will meet all your worries.
The only other thing I can think of , is for you to start up a different business altogether (hard given the work you've already put into your current venture), and tell NO ONE about it.
It won't quite be like starting anew, because you have gained a lot of business acumen in your journey so far, and those skills are invaluable, and transferable.
I probably can relate to you, as essentially, I am a very private person, and while I'm happy to see others succeed, I'd be the same as you are, that your private (as you thought) venture has been compromised by the emotional connection you have with your family... AND it's pronto even your brother that will benefit, coz if his relationship with his "new" (nine months is very little) breaks up, she'll take all that knowledge with her.
For all the reasons above, I feel you are NOT unreasonable to be upset, as I can see it cause even more problems than you've initially outlined in your op.
Hope you find a solution, and one that doesn't cause bad feelings.
I don't envy your tasks ahead in decision making.
Stay strong.