Your mother-in-law's assisted living costs are deductible. I do not see a requirement for an annual evaluation (of patient or facility) in Publication 502. (see "similar institution", below)
From Publication 502:
"You can include in medical expenses the cost of medical care in a nursing home, home for the aged, or similar institution, for yourself, your spouse, or your dependents. This includes the cost of meals and lodging in the home if a principal reason for being there is to get medical care.
Don't include the cost of meals and lodging if the reason for being in the home is personal. You can, however, include in medical expenses the part of the cost that is for medical or nursing care."
