Apparently, my sister bit me the day my parents brought me home. I obviously don't remember this, but it happened. Bringing home a new sibling is probably very frightening for a young child.
Lilly, the main character in Lilly's Purple Plastic Purse, is back in Julius, the Baby of the World. She was a great big sister before Julius was born, but she becomes a BIG troublemaker after he's born. She doesn't want to share her room, she is upset that her parents praise Julius for blowing spit bubbles and screaming (but they don't praise her for the same behavior), and she is intentionally mean to her new brother by waking him up during his naps and trying to scare him with her costumes. Lilly's parents try to show her that they care by giving her special privileges and attention, but that doesn't stop her behavior. Lilly ends up spending a LOT of time in her "Uncooperative Chair." Keep in mind that Lilly is a school-aged child, probably around 6-7 years old. She's not a toddler throwing a fit!
Everything changes, however, when they have a party for Julius. The entire family has gathered, including Lilly's Cousin Garland. Cousin Garland makes one big mistake during this party: she says a bunch of mean things about Julius. She says his eyes are beady, his nose is slimy, and his fur does not smell good. The reader isn't surprised by this because Lilly has said the exact same things to her parents about Julius. This time, however, Lilly reacts differently. She gets ANGRY. She picks Julius up, makes Cousin Garland take back her words, forces Julius into Cousin Garland's hands, and then makes Cousin Garland yell, "Julius is the baby of the world." These are the words her parents often say, but Lilly usually says, "Disgusting!" after her parents say it. Once Lilly hears someone outside of her nuclear family demeaning Julius, she realizes her mistake and becomes a great big sister again. In the end, only family can make fun of family!