How much sense does this make?

I meant to post this yesterday...

We go get the girls from school. Little J walks in and discovers that Baby D had gotten into her Tinkerbell cards. I sent Deidra up to their room to go get books to read for their read-a-thon and while she's up there Little J goes stomping and screaming into their room and walks up to Deidra and just bites her in the arm. Deidra starts crying and lets me know that Little J just bit her and sure enough, there's a good imprint on her arm. I ask Little J why she did that and she says "I'm mad. Baby D got into my stuff." I asked her what that had to do with Deidra. She just repeats "I'm mad." Deidra tries to be nice and went to help her pick up her cards and Little J scratches her. I send Little J down to time out while I put the boys to bed and she immediately starts playing the "Nobody cares about me" card. I sat down with her and told her that wasn't true and that she knew it and that she was doing a time out because she bit Deidra for no reason. I told her that we don't bite in this house. She says "well, I guess I'm just an idiot then, nobody cares about me, and I don't care if I hurt myself anymore." She starts banging her head against the wall. I told her "we do care about you here and deep down you know that and you need to be good to yourself. She stops and finishes her time out. She gets up and immediately went to Deidra and tells her "I love you and I'm sorry. I was just mad." I still don't understand though that she's mad at her brother, but she chooses to bite Deidra.

I think Deidra was just in the wrong place at the wrong time, but I wish I knew how to get this little girls anger in check. She's been doing so much better these past couple of weeks and then we get this today.

Update: After all of that, she just came to me on the couch and said "you are my best Mommy I ever had...you care about me and make me be good to everybody and nobody ever did that afore. I love you and Deidra and everyone here and I'm sorry for biting." I think we're making some headway now. Talk about making me cry.