They were made to be displayed!
I, too, have ivory carvings, more than 30 (the tallest are about 16" high). Mine are more than 100 years old and have been passed down to me from my late Mom. There are gorgeous carvings of Chinese women, balls-within-balls-within-balls, and other beautiful examples of intricate ivory carving. Most are elephant ivory, but I also have some made of walrus tusk. I also have quite a bit of ivory jewelry (some old, some brought back to the states by my Dad when he returned from his Air Force tours in Vietnam). I wear the newer jewelry, but keep the old stuff in a lovely display case.
I keep the carvings on display, but out of direct sunlight and away from our wood stove! Most are on display in our guest room, which has a blue, white and pale yellow, semi-Asian theme. I wipe them with distilled water and a clean cloth monthly (drying them right away, too). I always wear cotton gloves when I do handle them. Once a year, I give them a light rubbing with beeswax, to protect and moisturize them, buffing well to remove the excess.
We have low humidity where we live. But, with the proper continued care, they are in wonderful condition.
I say display them (and care for them safely and correctly), so that you can enjoy seeing them daily. I think this would pleased you gm in-law, as well!
If you have any concerns as to their value and condition, contact the museum in Portland (they have a rather nice ivory collection, as I recall). The Smithsonian would also be a good contact as to their care, if you need more info!
One other thing, take photos of them for your insurance records and document their history (ask your gm to write it out for you)!!