Out of control teenage son?

My 13 year old son (going 14 on May 5th) is out of control. Today when my children came back from school, my son starts yelling at me for doing absolutely nothing. He has no respect for his family members, ESPECIALLY his 11 year old sister. My daughter always looks out for me and defends me from my son. While he was yelling, she respectfully told him that it was immature to scream at his mom (me) and to stop. He then approaches her kicking and hitting her while swearing before I got to stop them. When his sister walked upstairs and yelled "This is why I hate you and why you have no friends!" he then runs and grabs her and starts to again, hurt her physically and mentally. What ever punishment he gets, it doesn't stand in his way to hurt her. This happens 5-7 times a week to her, and I hate seeing her like this. After this happens, she always threatens us that she will kill herself, and I don't want that to happen. What should I do?