Yevamos 10:6-7

Yevamos 10:6

If a boy of nine years and a day performs yibum, he disqualifies the widow to his brothers. They likewise disqualify her to him except that the minor only disqualifies her at the start, while his brothers disqualify her both at the start and at the end, as follows: If the minor performed yibum, the brothers may no longer marry the widow. If one of the adult brothers performs yibum or maamar with the widow, or gives her a get or performs chalitzah, the minor may no longer marry the widow.

Yevamos 10:7

If one minor brother of nine years and a day performed yibum, then another minor brother of the same age did likewise, the latter disqualifies the widow for the former; Rabbi Shimon says he does not disqualify her.