Answer: After considering the obvious
and discovering they don't work, consider this configuration: a sister
and brother, each of which is married to someone not present, with the
sister's daughter and the brother's son (or sister's son and brother's
daughter). In that case, the sister is sister to the brother and
mother to the daughter. The daughter is cousin to the son and niece
to the brother, who is of course uncle to her, and so on.