I understand exactly how you feel Op, my DB was given money monthly by our mum (he didn’t work) plus a large 5 figure lump sum. All kept secret from me but not from other relatives.
I found out by accident when mum was ill and we were managing her finances. Her answer - “You never asked for anything!”...it did explain some comments she blurted out once or twice! My dad would have never given one of us a lump sum and not other, not sure about monthly money.
At least you know - I found the secrecy so upsetting but know that DB threatened her to keep her silence - he didn’t want me to have the same and reduce his inheritance. My cousins told me about a letter her wrote to mum after I found out, calling her every name under the sun and telling her he wanted it all, he would make her change the will.
I did get a lump sum, just months before she died, but it felt like tainted money and most has been put into my DCs’ ISAs.
I lost all respect for her (never had any for him) and had I found out earlier, would have gone NC and left them to it. My cousins said they always felt she treated me worse than him, I never felt like that growing up but she was always more critical of me and he was definitely the golden child.
Weeks after I found out, we were told she was terminally ill, and I knew I would ultimately regret walking away, so I did what was right for me - helping to look after her in her final months.
DB was nasty then, and after she died, but we are now LC and I would be quite happy if I never see him again. He is eating his way through his inheritance now his handouts have stopped. He knows never to ask me for help!
I would say something if I was you, then you can make up your mind on how you react. Mum couldn’t have managed without my help, DB lived further away and didn’t drive, but that seemed irrelevant to them.
I would never treat my 2 differently. They don’t get exactly the same help to a penny, but are treated as equally as possible.