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.