Gah this is a difficult one op!
On the one hand, in principle, I am convinced that parents should be strictly fair when it comes to Time, money, attention etc towards their adult dc, even if the dc are living in different financial circumstances, that is, they all get the same even if one is a sahm married to a millionaire and another is single, earning a minimum wage.
This is because money, time and attention become interpreted as love, and if you don’t ration those things out exactly the same, then adult dc tend to think that their parents level of love for them is not the same either. I’ve seen families really pulled apart by this.
However, the minute serious illness rears it’s ugly head, whether that be physical or mental ill health, which negatively impacts on earning potential, then things may need to change out of necessity. For example, at one extreme, if an adult child has a serious car accident involving a spinal injury, then it’s obvious more family resources will be poured in to them far exceeding what the other siblings receive.
Here though op, you don’t know if this is a case of “golden child” or if your parent’s interventions are justified, in other words, you may not be privy to the extent of your brother’s mh issues, or if your brother is exploiting your parents?
Even if you do know what you are dealing with, beyond having a discreet word with your parents which has not gone down well, what can you do? It’s their money and they are of sound mind. That’s a huge amount of money to give away though!
I can understand why you feel hurt and I would advise focusing on your own career and earning potential because it sounds like your parents will not have much money left to leave you in their will. And certainly get proper financial advice when that time comes.
This doesn’t sound at all fair 🌷