Hi,
yes, your grandfather Marcin Polny and his brother Adam Polny were from city Nisko. See Passenger Records and Manifests;
Marcin;
http://www.ellisisland.org/search/passRecord.asp?pID=1022080...Adam;
http://www.ellisisland.org/search/shipManifest.asp?pID=10241...I found in phone book Polny's in near city Stalowa Wola, but not in Nisko.
Cordially,
Wladyslaw