first let me say I am not an official appraiser - and - its my personal feelings - since they have been in the family for that long -I truly hope they stay there.
the extentions on the 12 and the chip by the tang cause some value issues and you want to be sure the wood is in good enough condition to shoot- the value is certainly seriously affected. you might want to watch ebay or the gun auction sites and find new wood for it.
now - on the other hand - a 28 inch 0 frame 16 gauge in good shape - as yours looks to be - is a desirable gun - way more than $500- check Gunbroker.com or gunsinternational to see the value to insure it for. shooting light loads like RSTs (
http://www.rstshells.com/ ) and keeping it clean will let you see the 5th generation shoot that gun.