One way this is possible is through inbreeding. A niece is a sibling's child, and cousin is a parent's sibling's child. So if person *A*'s parent and sibling have relations with each other and share a child *B*, *B* is both cousin and sibling to *A*.

Another way this is possible is through adoption, because when someone is adopted, they can be considered the "child" of both their adoptive and biological parents. If person *A*'s sibling *B* gets adopted by their grandparents,and then has a child *C*, *C* is both cousin and sibling to *A*, since *B* is both *A*'s sibling and parent's sibling.
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