Bullys

[N723GW]

My girlfriend and I share a home, and we have a bully. She has two kids, a 7 year old and a 3 year old. I have a 3 year old. All girls . Her 3 year old is a bully. In one day, she whipped a book at my kid, pulled her hair and kicked her in the legs, knocking her down. She is constantly snatching the other kids' toys and laughs about it, and when we put her in time out (a spot in the living room) she squirms and gets up and runs around. No matter how many times we put her there, the last resort is putting her in her bed and closing the door to her room (which she shares) . She can be very sweet, but then very mean. Her dad and step mom claim they do the same thing with the punishment, but it is just not getting through. What do we do?!