My little mutt Casey was my only "baby" before Rachel was born, so it is safe to say we were a little concerned how this spoiled 15 pound mutt would react to a new "sister". At first she wouldn't come within 10 feet of her. Then after a few weeks she would come and sniff her briefly but would quickly leave. Then she would come up, sniff, and stay a while.
I was sitting on the sofa the other day, Rachel's head in my lap and her butt on the sofa. Casey, my little mutt, usually sits to the right of me on the sofa, where Rachels legs were. Casey jumped on the sofa, sniffs Rachel's legs and starts to take her nose and flick her blanket on her legs. She must have spent a full 5 minutes trying to cover her, and actually successfully did. It was rather interesting to watch. Since I took Animal Behavior in college and never really used it, I figured I'd humor myself by saying I'm using it here and she's now becoming "motherly".
So I figure (I'm into multiple choice these days):
A) My animal behavior class was worth the money and I accurately protrayed Casey as mothering Rachel because in the wild the Alpha dog has all the puppies and the other members of the pack help her raise them.
B) Her diaper was dirty and Casey didn't want to smell it anymore so she was trying to bury it.
C) She is actually in a jealous rage and was trying to smother her.
I prefer to think "A".