It's not your money. It belongs to your parents and it's their right to spend it how they want. Maybe your brother is the favourite for a reason?
However, you need to decide what you want.
The money has been already been promised/given to your brother. And they don't have enough spare cash to give you a gift of equal value.
However, you do have "emotional only" grounds to make a case about future inheritance.
£90K this year is not worth as much as £90K in twenty or thirty years.
You could ask that if you are not being given anything to improve your quality of life while you're young, that you are given £110K or £120K before the inheritance is split, when the time comes.
You could ask for 10K now, so that your quality of life is improved right now with a new bathroom - but if you continue with a sulky entitled attitude, I wouldn't give it to you.
You could also remember that you were probably delighted when your parents stumped up £5K towards your wedding. You're hardly Cinderella in rags here.
So I would write your parents a very loving letter, explaining that you feel hurt and belittled by their generous offer to improve your brother's young life and make him secure - but you would hope they would want to be as generous to you, when they can.
And if they don't want to be, you either have to suck it up or be a brat and lose your relationship wiht your parents. Sulky and being outraged does not make you more endearing to them.