Who is Dora’s brother? Asked by Josie Orr Guillermo Márquez is Dora’s baby brother. He is a 1 year old baby who has a twin sister named Isabella. He wears a blue shirt and a white diaper. Robert Curry, BA Contributor 4.1/517 ratings Rate this answer Leave your comment You may be interested inWhos dora the explorer's cousin? Diego Marquez, Dora's cousin, is an 8-year-old action-adventure hero who loves nature and animals.Who is Dora the Explorer's cousin? Meet Diego, Dora's cool cousin! Diego can talk to animals, and he rescues them when they're in danger.