We’re always most jealous of the youngest, but actually it’s better for your mind and happiness later in life to be able to handle the world better, and growing up too safe and favorites can definitely prevent that.
We have 2 kids, and the oldest feels jealous when I give the youngest focused attention. I constantly have to remind her that I did the same with her, and it's only fair. But I can see how it's tough on her.
This week our 4 year old wanted to try swimming like her sister without floaties. So I was helping her navigate the water where she had to keep herself up, and my oldest (7) was definitely feeling jelly.