The scanner/copy idea others mentioned is good, but I would ask some local professional photographers if they would be willing to help.

Depending on the type or thickness glass, it might be difficult for a scanner to get a clear image, in which case a DSLR camera might be able to copy it better. A photographer, especially one who knows art reproductions/documentation would be able to re-photograph it without glare.

Photographers understand the deeply personal value of photographs like this, I’m sure you could find one who would volunteer to help or do it for a small fee.