How many letters does the top answer for “the granddaughter of one's brother or sister.” have?

The most common answer is grandniece (10 letters).

Crossword clue: the granddaughter of one's brother or sister.

1 answer found in Letterora's clue database (Wikidata facts, Open Trivia DB, WordNet and Webster definitions — all licensed sources).

grandniece 10 letters web-def

a daughter of your niece or nephew

the grandson of one's brother or sister. 60% similar → grandnephew