Guys,please help

So my family is pretty much a little messed up.But what annoys me most is when my dad starts getting really really aggressive with my sister.She would start crying and he would start yelling at her,or trying to drag her up the stairs and sending her to her room saying,"There cry all you want,I won't care and now you won't bug/bother anyone else." Then he would leave her alone in her room and she'd start hysterically crying.And there is nothing I can do