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Wentworth is Tiffany's only younger sibling, and we mostly see him through the prism of Tiffany's life. To Tiffany, he's a huge annoyance—he showed up out of nowhere, stole her place as the beloved youngest child, and on top of that had the audacity to be the only boy in the family. Wentworth, quite frankly, isn't Tiffany's favorite person in the world.
He's also very sticky, which is quite possibly the word most often associated with him throughout the book:
Anything could make Wentworth Sticky. Washed and dried and left in the middle of a clean floor for five minutes, Wentworth would be sticky. (1.16)
So Wentworth is basically a gross, sticky baby who steals all the attention. Even so, when he goes missing Tiffany runs out to save him. She may not like him all that much, but he's still her little brother, and she'll get that sticky bundle of joy back to her parents no matter what she has to do. He might be a sticky mess, but he's their sticky mess, after all, and home is where he belongs.
Through dealing with Wentworth Tiffany realizes that what's important isn't whether or not you like someone—when you're in a family, you're responsible for each other. You take care of each other and make sure that you're all safe. And that's exactly what Tiffany sets out to do—even if it means braving danger to rescue her least favorite family member.