How far will you let you children fight?

When will you step in? I remember saying some really awful things to my sister and i remember my mom just reading a book well me and my sister screamed at each other! My sister told my mom i was out at a party and she called me a Whore and a bitch and i remember yelling at her "did it hurt when you had your fucking baby?!" and my mom didn't step in at all. At what point do you step in your children argument or fights