Here is a short mention of the WWI organization, but not much data. more on WWII
Pennsylvania State Guard - Wikipedia
Here is where you can type in your relatives name and see if it comes up as a state guard man from 1918 to 1920
https://archon.klnpa.org/psa/?p=coll...olcard&id=8760
There are more records and organizations involved in home defense than the state guard: here are the state archives, you might find a lead there.
WWI Online :: Pennsylvania State Archives
I would document what you can find, in writing before it is all lost. Thing is family firearms have a history of losing their association or having the story get messed up over time. Have the documentation and it will stay a family heirloom a lot longer.
For example, in my family is a
Belgium
muzzle loading screw-off double barreled percussion pistol from circa 1851~1855. It has family history, but it is forgotten, the last person who knew something about it died in 1993, and his widow had no recollection of what the association was. The best guess is it was from an extended family from Indiana circa civil war era, but no one knows.