why did the mice eat the baby mice?

[feebs83]

I have 3 mice a mother and her baby and a male. The 2 females are both pregnant. The mother mouse just had babies and the other female (her older baby) just ate the one of the newborns. OMG WHY? i'm so upset and sick right now. I grabbed her when i seen she was doing it and she ran away and started to eat another but i stopped her and put her and the male into a box. I dont know if she had eaten any more. I took the dead baby out and put all the other scattered ones in a pile and put some of her nest over them but she just took them all over to the corner and rolled them in the bedding. Why dod all this happen? could the baby be jealous of the newborns.. and will she eat her babies when she has them? Thanx peoples xx