sisterson
The son of one's sister; sororal nephew
rare
But next year, to conserve the soil, it'll be put in alfalfa, and my sisterson Willy takes care of it then.
Where it comes from
From Middle English systerson, sustersone, from Old English sweostorsunu, from Proto-West Germanic *swestersunu, from Proto-Germanic *swestērsunuz, equivalent to sister + son.
Adapted from the Wiktionary entry sisterson (CC BY-SA 4.0).