nephew

A nephew is someone's child, usually male, whose parent is their sibling. This person shares one or more grandparents with them.

If your cousin has kids, they might have nephews and nieces for you. You'd call those little cousins by those names because of how they're related to you through family ties. For example, if your brother's son is playing at your house, that boy would be your nephew. If your sister marries someone with children from a previous relationship, their kids might become your step-nephews or step-nieces, depending on their age and sex.