What is a grandchild call if not sibling?

If a grandchild is not a sibling, they can be referred to as a cousin, if they share a set of grandparents with another child. Alternatively, they can simply be identified by their relationship to the grandparent, such as “grandson” or “granddaughter.” In familial contexts, terms like "nephew" or "niece" could also apply if the grandchild is the child of a sibling of the individual in question.