You need to tell your mom that you've been feeling left-out, and loved less than your brother. She may at first say that you are being silly, or that no such thing is happening (because she may truly not have noticed that she is treating you differently), but tell her your feelings are real, and you've been hurting, and point out specifics if you can as to when you felt "proof" of this awful feeling.
I hope that she can understand your feelings, and that things get worked out for you soon. Don't let these feelings build up inside of you.
-Erin-