Child abuse

I have a 25 year old sister that has a 1 year old and a 9 month old . And everytime the baby daddy makes my sister angry she takes it put on my neices. Today they were arguing and he walked out the door while my sister was talking and that made her very angry so instead of chasing after him or letting him be she turns around and slaps the 1 year old k. The face twice until she fell on the floor crying and then the 9 month old started crying she slapped her in the face too and then she started hysterically crying herself saying that her baby daddy made her do it .and then as I'm trying to calm her down. The 1 year old walks over to us and she slaps the little girls again in the face. The side of her face was very red and I left an hour ago after calming things down a but but now I'm not sure if I should call the police. Advice please

Edit: I called the police and 2 officers showed up to do a welfare check but nothing came of it .the children are still with her.will call cps in the morning.