This is really about family Dynamics not age... This is a very common problem in families..

I dont think anyone needs to just roll over and take whatever crap is thrown at them. I dont know if you see a Therapist but I suggest you do.. A Therapist can help you learn how to set boundaries and keep them up when someone is trying to roll right over them. Keeping them up can be a hard thing to learn but it can be done.

Its doubtful your Mother will ever change and be a Mom you wish she could be. You will have to learn which battles is worth the energy you will spend and which ones you will learn to let roll off your back. Life just isnt fair.. My Mother favored my brother. I have no idea why, she just did.

Be kind to yourself and maybe start writing down things that she does that anger you.. You will likely find key issues that you clash, then you choose what your just not going to tolerate and be forceful is telling her your will not tolerate X any longer and that you will just get up and leave and do so. Will she get mad Yes ! But so what, let her get mad, she will either get over and act nicer or she will consistently watch you walk out the door.As she learns you just wont deal with her about X then add another thing your just not going to deal with.... Boundaries?they are hard to build and keep strong but everytime you dont allow someone to knock them down they get stronger.

You really have no control over what your Brother does or doesnt do, sad. But you can control what you tolerate and do.

Good luck