Inheritance is so variable that without knowledge of specific diagnosis and genetic cause for that diagnosis, there is no way to know if your grandchildren have any risk. The fact you know that your wife's grandmother had Alz needs to be entered in the family medical history, for future reference.
My husband's maternal grandmother (in 70's) & mother ( 60's) passed away from dementia...at a much older age than my husband (58) is now. My husband has an older brother with no problems at this time. We have 2 sons, however, they don't dwell on what might happen in the future.