How do you get a defiant 4-yr old stepson to listen to you?

So far the responses to me asking him to pick up a toy, or go brush his teeth, or whatever, have been as follows: 1) Clench his fists and run up and head me on the leg, 2) Scream and growl at me and tell me I'm not his anymore, 3) Deliberately walks off and shrugs his shoulders saying he doesn't have to. I've been here for over a year. His older sister is even worse but the mama tells me she's a lost cause. The mama also says the boy never acted like this before I got here. He defies her as well, and his tantrums are legendary in our apartment complex.