Not as terrifying as your "English"LOL
@Hi, Lenko! Thanks for catching that one.
Haste makes waste, right? Had I thought for another second, I'd have realized instantly that the GENITIVE SINGULAR MASCULINE of siostrzANIEC is in fact "siostrzeŃca", and WITH a soft sign as written, therefore, of course NEPHEW:-)
A beginner mistake I oughtn't have made, particularly as a professional translator!