Step-brothers, step-sisters, step-mothers and step-fathers exist, as do foster parents, foster sibling, legal guardians, etc. Some kids' mother or father died, or they're divorced, estranged, etc.
Using the perspective from the OP, it's a kids' book, using a couple of different synonyms might help them better understand what they're reading and not walk away believing that there's something wrong with their family or themselves.
Using the perspective from the OP, it's a kids' book, using a couple of different synonyms might help them better understand what they're reading and not walk away believing that there's something wrong with their family or themselves.