Who is the antagonist in Smiles to Go by Jerry Spinelli?

Will's sister, Tabby, is the primary antagonist, though not in the sense you might think. She is portrayed throughout most of the narrative as being extraordinarily troublesome and pesky, seemingly taking every opportunity to mischievously tease, taunt, and otherwise irritate her big brother. Will's mother later reveals that the only reason Tabby puts so much time and energy into irritating her brother is that she wants to be like him and wants him to love her. When Will discovers proof of this, he begins to change his mind about his sister, his feelings and attitudes towards her subsequently starting to change.