Haha! The lowest grade I ever got was when my oldest brother was helping me in chemistry.
I wonder how much of it has to do with interest? I love to read, but was bored with math, my brother loves numbers and statistics, but hates to read.
I'm the youngest and I got the best grades in english and had no interest in math (I still got good grades though). While my oldest brother had no interested in english (still passed, but my math grades were higher) and got the best grades in math.