You did not say what language these letters were written in. We had a photo of my husband's grandfather with two people which was translated as "nephews" Knowing that this was
probably not true, we inverstigated further. What was originally written was in Yiddish and said "Schwester sohn" This is litterally ;"Sister's son" which would be a nephew.
However, we found out from an expert at YIVO that this was old terminology that was, in fact, used for a cousin.
Hope this helps. Stay well.
Avivah R. Z. Pinski
near Philadelphia, USA
Researching: Zuchman in Sarnaki, Karczew, Warsaw Poland
Reznik in Drohiczyn nad Bugiem, Siemiatische, Siedlce Poland
Rifczes in Lemberg, Vienna
Kopekin in Polatsk,& Besonkovich in Belarus
Familiant & Koifman in Bessarabia and Ukraine
Sondak in Vitebsk, Belarus and Rehitza, Latvia
Aginsky and Slonimsky in Minsk
Aronofsky in Belarus & Lithuania