What's the Name of That Book??? discussion

I read it as a child (likely middle-grade level), so very late 1990s or early 2000s.

Contemporary setting, not fantasy or historical.

Standalone book (probably not part of a series).

Main character is an older sibling, who believes the younger sibling (likely a girl) does not talk.

The sibling does talk normally to others outside the house, including in a library—this is where the protagonist first notices. The little kid is at storytime and is talking normally to others. The librarian mentions something about the child talking to the older sibling, but they are shocked and do not believe it at first until they hear it for themself.

The older sibling encourages the younger one to reveal they can talk at Christmas or other major event. Once the sibling does speak in the big reveal, the child is a chatterbox.

I think the speech plot is a minor or side storyline, not the central focus.

Tone likely warm, emotional, or heartwarming—not high drama or comedy-heavy.

It might not be on this Selective Mutism list if it's a minor storyling, but might be worth checking anyway.