Hi NLgarden,
I too think your mom-in-law's plant looks like Dieffenbachia and what a beautiful specimen ... she has obviously taken great care of that beauty to have it thrive for so many years!
We have quite a few Dieffenbachia listed in our database with info and photos: :
http://garden.org/plants/searc... and they would all take the same care.
I sure wouldn't changes a thing regarding the care, it really is gorgeous and it's obvious it's been in good hands all these years!
If it needs a larger container just tell her to be sure the pot has good drainage and to use a good quality compost/potting soil.
