I'm not related and I assume you mean the Rushe (Rasia, Rusia, Rucia) Family of Allegheny County, PA. Without knowing exactly where in Alsace they were from it becomes very hard to find them there.
If you want to find where someone was from the best place to start is often where they ended up. If you haven't already I suggest getting Nicholas' death certificate and obituary. If you are lucky either one will list his exact place of birth. You might also try to get the death certificates of John and Christina's other children. If you're really lucky one of them might have the exact town John or Christina was born in. It's a long shot but you only need one to have the town listed.
If you know what church they belonged to find out what might be in their records on this family. If you know where they are buried check with the cemetery office to see what they may have in their records. Sometimes an exact location is listed in their burial records.
Based on the 1850 census it looks like there were other members of this family living next to and with John and Nicholas, notably Mary 50 and Nicolas 22. Finding data on them might also help.
BTW I believe Nicolas died Jan 31, 1942 in the Pittsburgh area. Keep in mind that John will be listed in French civil registration records as Jean although his real name may have been Johannes.