Talk to your mom about it and tell her what she is doing, and tell her how you feel about it.
My mom favors my sister over me, what should I do?
Sometimes what we perceive isn't the actual reality. I know I felt like that for such a long time growing up, and one day talking to my brother about it, he said he always felt the same about me, that I was the favorite. Maybe just be open about how you feel and take your mom out for coffee and talk to her about it. Her "answers" might just end up surprising you.