Babysitting nephew who teaches daughter bad habits.Advice?

R - posted on 07/19/2011 ( 1 mom has responded )

My daughter, Daphne, is 9 months old and loves her cousin, Johnny, who is 20 months. My sister-in-law is raising him on her own, which gives me great respect for her. However, her methods of parenting are quite the opposite of mine, and it's obvious since I've been babysitting him five days a week for the past month or so.
When he's getting constant, undivided attention from me he is generally sweet and well-behaved...on a good day. However when I can't be hovering over him, he starts lashing out. On a bad day, he often hits my daughter on the head or bites her. He will screech loudly to get a laugh, and when I tell him it's not funny, he bites me. Often throughout every day, he will snatch a toy out of Daphne's hand. She usually doesn't mind though, and is happy to see how he plays with it. When things aren't going his way, he throws himself down, doing a kind of backwards dive and smashing the back of his head on the ground. He often slams his forehead into walls or doors. He thinks it's funny to hit himself in the head. He shouts, "GO!" to Daph when she crawls to him, and frequently screeches "NOO!" in her face for seemingly no reason after a while of playing nice.
I've tried ignoring the behavior, leaving the room with Daphne, and having my husband pick him up and take him to time out. All of these allow the situation to escalate. The first week or so of watching him, using my face and verbal tone to communicate that none of this is okay was enough, but now he laughs at this.
Some of his behaviors that I believe are rubbing of on my LO:
When she's very unhappy (teething pain/inability to sleep/etc), she throws herself backward out of our arms, almost killing herself!!
She used to show me things she found on the floor and bask in the praise; now she hides them behind her back or in her mouth and crawls away at high speeds.
She hits me -hard!- and laughs when I tell her it's not nice.
She thinks it's funny when I try communicate to her something is not okay with my facial expressions, when before it upset her when I looked upset.

Since he's been here, he's learned to say, "Up?" instead of whining loudly and crying. He runs to greet me and I love taking him up into my arms and hugging him. He's gradually become much less violent, but only when things are going his way. We have so much fun in the bath, and we all love each other very much, so I don't want to have to give up watching him. Any advice at all??