How many letters does the top answer for “the brother of your father or mother; the husband of your aunt” have?

The most common answer is uncle (5 letters).

Crossword clue: the brother of your father or mother; the husband of your aunt

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

uncle 5 letters wn-def

the brother of your father or mother; the husband of your aunt

an aunt of your father or mother 67% similar → grandaunt

the father of your father or mother 50% similar → gramps, grandad, granddad

the mother of your father or mother 50% similar → gran, grandma, grandmother