stepcousin

A nephew or niece of someone's stepparent; a child of someone's stepaunt or stepuncle (in the sense of a sibling of someone's stepparent).

A stepchild of someone's aunt or uncle; a stepnephew or stepniece (in the sense of a stepchild of someone's sibling) of someone's parent.

A child of someone's stepaunt or stepuncle (in the sense of a stepsibling of someone's parent); a stepnephew or stepniece (in the sense of a child of someone's stepsibling) of someone's parent.