I suggest family counseling, at least with you and your sister, to see if you can't improve the dynamic. Frankly, I don't blame your sister for hating your mom--not only did your mom fail utterly to protect her from abuse, she abused her herself.
And honestly, if my mom hit someone who was holding a baby, my mom would not get to be around my baby unless they had taken anger management therapy. Period. I'd focus on working on your relationship with your sister. Unless your mom has a drastic change of heart, that relationship is beyond repair.