Do you have a record that shows evidence that Alois Fliess was Jewish? Or is this something that you have heard from a family member, that Alois had a Jewish background?
For checking Polish Jewish records that have extracts into English, you can look on the JRI-Poland website. You can choose either Prussia or Galicia to check German or Austro-Hungarian records.
If I were you, I would still look into Polish Catholic records for both Alois and Wilhelm. There are probably Facebook groups in the English language that specialize in Polish genealogy, that can advise you how to do this.
Lak/Lok/Liak/Lock and Kalon/Kolon in Zagare/Joniskis/Gruzdziai, Lithuania
Lak/Lok/Liak/Lock in Plunge/Telsiai in Lithuania
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