My MIL can be like this. She has her “choosen" people in my family, her son and our first (of 3) daughter. Here's a perfect example.

One year for Christmas I had asked for hair supplies for all 3 girls. Each daughter recieved headbands, hair ties, and barrettes for their hair type and individual fashion type. After breakfast and gifts I was picking up a little. I saw her “helping” which was not unusual, until I spotted her rummaging through the younger girls piles and “re organizing” them. The next day I tried to help the littles with their hair. They were excited for their new accessories! Well I could not find any of their things. Turns out my MIL had taken all the hair supplies from my other two daughters and gave it all, 100%, to her favorite granddaughter. She wasn't even the one who purchased it all, in fact several people did including my family and friends too.

My solution….now no one has their own hair supplies. I do not allow it. Everything goes into a central location we all have acess to. That's how we deal with her. Whenever she singles out a pet in the family we smile and nod and when she's out of sight we share. No one should be able to put that wedge in our family. Just saying.