My Dad Is My Uncle's Brother by Joe Lyward

A family is a thing of many parts – a complicated puzzle made of brothers, sisters, mums, dads, uncles, friends, grandparents. Children can often get lost in the maze of kinship, trying to understand who is a great-uncle and a stepfather, who's married to whom, what in-laws are, and why someone who's only just been born can immediately become a niece or a nephew to someone. Together with his father, the protagonist of this charming book climbs his own genealogical tree, happily finding his own place as a part of this big, lively, crowded universe.