Stealing Kittens

My mom has two female cats (they are both sisters) well they both got pregnant  by my Male cat (who is there brother) (who is also fixed now) one of the cats had her litter three weeks before the second cat but my stepfather was a jerk and disposed of the day old kittens.  Now the second kitty had Nine babies but one died shortly after birth. But our concern is the first momma cat is stealing second mommas cats kittens batting them around like mice and stealing them.. what can we do to prevent this?

You need to separate them better, something like a room with a door would work best.  You shouldn't let kittens just have a spot in a living room etc if there are other cats roaming around. I always dedicate my whole spare bedroom to the kittens whenever we have a litter. I know you said the male is fixed now but in the future avoid inbreeding cats, most inbreed kittens will have health problems when they are born and normally don't live that long. 

Disposed of the kittens? Poor kittens! Poor mama kitty to begin nursing and have her babies taken from her.

However, you'll need to keep her away from this mom cat and her kittens since she is mistreating the kittens. She's clearly confused, but your remaining kittens will not survive otherwise. Like has already been mentioned, you'll need to keep mom & kittens in your room with door closed and the other kitty stays out for now.

You need to have both your females spayed.