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.