Julius, the Baby of the World: 12/19/15
Julius, the Baby of the World by Kevin Henkes is about Lilly's undying disgust for her baby brother. Their parents, though, are completely gaga for Julius, calling him the "baby of the world." Gag.
Of course in the end, Lilly realizes just how much Julius means to her but it takes a cousin calling him yucky to do that. He maybe an annoying, slobbery baby, but he's her slobbering baby brother. And only she can tease him that way. Or something.
I'm a big sister. My husband is a big brother. We have two children. So we know both ends of what it's like with the dynamics between older and younger children. It's natural to feel some jealousy and frustration and one's younger sibling but I have to agree with Lilly here. Her parents are being absolutely awful, showing very definite favorites for Julius.
For a better version of the same type of scene, I recommend Wee Free Men by Terry Pratchett in which Tiffany Aching ends up rescuing her annoying younger brother who is annoying all on his own without parental meddling. Another good example is the movie Labyrinth in which Sarah has to rescue her brother from the Goblin king.