Now, don't take as disrespectful.
The Spreewerke's are the least valuable when it comes the WWII P-38's. They are still very collectible, and yours has more value to your family than one that anyone could ever buy. That being said I would shoot it and enjoy it. Since it will hopefully never leave your family I would not worry about reducing the value by shooting it.
The thing I enjoy most about shooting my post-war P-1 is watching the people on my left. In case you haven't noticed, the extractor is on the left of the slide and the brass ejects to the left. People standing to the left of you will not be expecting to get hit with flying brass.
Shoot it. Enjoy it. And may it always bring you fond memories of your grandfather-in-law.