If they are dead ....
The son/daughter may have the right to perform a record request (Personal Act request/not FOIA) with DESPP and ask to inspect any gun records in their name....I would imagine that any gun, if "registered" would be under the male's name. Registration of the gun would have been voluntary.
Under the new PA13-3 and PA13-220 ... you may find answers
Do you have a carry permit or eligibility certificate? One of these may be "needed" under the law ...
Or did your in-laws gift you the gun prior to PA13-3 being passed? Then PA13-3 / PA13-220 would not apply...and you would have to look at the law prior to the passage of those two abortions.