wow. That is really rude on their part. Especially with them being celebrities and being so public and vocal about their 'favorite.' Yuck.
Our two kids have a running 'joke' about the other child being the favorite. Our son thinks his baby sister is the favorite, since she is a girl and is the youngest and is very beautiful. And spoiled. Our girl thinks her big brother is the favorite because he is the oldest, the most responsible and a male. We always agree with them when one says.. " He/She is your favorite. " We say YES, clearly, he/she is our favorite. " But I think they both realize that each is a 'favorite' in a different way. Each has their own valuable attributes and character traits and we love them each for different reasons I suppose. But neither 'outranks' the other.
But for them to label one as their clear 'favorite', at such a young age, that is just wrong, imo. They are asking for future trouble, imo.