My step-dad hit my baby sister?

First off I don't know who to tell so thats why im asking a bunch of random people... Anways I just walked into my house where my 1 year old baby sister and step-dad are. I don't think he heard me come him cause he was yelling. At first I thought he was talking to the TV like he always does, but I walked upstairs quietly and saw him slap my babysister across the face HARD, like he was hitting a MAN. Then he pushed her on her back, and put his hands on his head and took a deep breath. Then he sat her up as she was crying furiously and then he told her to shut up, and spit in her face. Then he started changing her diaper (I sat there to make sure he didnt touch her inappropiatly, cause that would have been my last straw.) Then I quietly ran downstairs, opened the front door walked out then came back in and slammed the door. He popped out the room and said "Oh you scared me" Walked back, picked up Laura and came downstairs (as she was still crying) and shoved her in my arms, then said "Take her im going to take a nap". I ran to my room with her and inspected her all over, she wasn't marked with bruises or anything. OK. Please I reallyyyyyyyyyy need help! PLEASE im not trolling, I just wanted to make it as detailed as possible because I wanted everyone to FULLY understand what happend and what I saw. I can't tell my mom cause shes head over heels in love with him, and even when I tried to tell her 2 months ago that he snatched the remote from my hands and pushed me out the chair, she didnt believe me, so I KNOW for a fact she wont wanna hear or believe this. I have cheerleading practice everything Thursday (today) and Mondays, which is when he has her all to himself. Other then that I get there before he does, and im thinking of quitting so he won't do that to her anymore, cause only god knows how many times hes hit her before. But who should I tell? Hes trash, my mom refuses to realize that hes trash, I have no other family except my grandmom whos in a NURSING HOME (he put her there) and I refuse to call CPS or police because splitting her up from me isnt a option. What should I do to keep her safe. Its only a matter of time before he starts hitting on me and my mother too...