Ask any mother of multiple children who her favorite is, and she always rebuff you. "I love ALL of my children equally, OF COURSE." Bitch is LYING. Yes, mothers all love their children, but they don't love them equally. It's just a fact. She's only human. Just like you have a favorite co-worker or a favorite cousin, your mother has a favorite kid. She can't help it. Out of your siblings, one of you is bound to be more simpatico with your mother. One of you has more similar interests, calls her more, visits her more, whatever it is.
So, if you know you're the favorite, get her this Mother's Favorite Card ($6, Moglea). It'll piss off all your siblings, which is always the goal, right?