You can comb through US records to discover more about the ethnic background of the aunt Sophie, nephew Leonid, and other family members. Most of the census records for 1930 and earlier ask about the native language a person speaks, as well as the language of their parents (whether the parents immigrated to the US or not). For Eastern European Jews of that time period, the language would be Yiddish, rather than Russian or Polish, etc. On ship passenger lists, "Hebrew" was the ethnic classification for Jews (which was called 'race' at the time).
In the event that your nephew decides to go the DNA route, it's best to test the oldest descendents of the sisters Nadejda and Sophia, since those descendents will have more of their DNA. If you test at 23 and me, the Jewish DNA category is called by that company 'Ashkenazi Jewish'. On AncestryDNA, the Jewish DNA category is called 'European Jewish', and is basically equivalent to Ashkenazi Jewish.
Lak/Lok/Liak/Lock and Kalon/Kolon in Zagare/Joniskis/Gruzdziai, Lithuania
Lak/Lok/Liak/Lock in Plunge/Telsiai in Lithuania
Rabinowitz in Papile, Lithuania and Riga, Latvia
Trisinsky/Trushinsky/Sturisky and Leybman in Dotnuva, Lithuania
Olitsky in Alytus, Suwalki, Poland/Lithuania
Gutman/Goodman in Czestochowa, Poland
Lavine/Lev/Lew in Trenton, New Jersey and Lida/Vilna gub., Belarus