i just wanted to add, that i work with kids... a lot of teens act this way. i did, and the more parents tried to tell me what to do, the worse it got.( i do not have antisocial personality disorder... )
without offending you, i hope i can offer the suggestion of taking parenting classes. i am not implying that you and your cousin are bad caregivers, but you seem overwhelmed, and unsure of how to handle her behavior.