There are no medications available today which slow memory loss over an extended period of time. The medications approved for Alzheimer's have been shown in controlled studies to slow memory loss over a few months, but after 6-12 months, memory decline in those who take the medications is similar to those who do not take the medications. New drugs are being studied, and hopefully one or more will demonstrate a clear ability to reduce or stop the decline in memory impairment (we should not expect any of the drugs to restore memory loss).
It is always a good plan to have your father-in-law examined by an experienced geriatrician or geriatric psychiatrist when signs of memory loss emerge. The reason is that memory loss may not be due to Alzheimer's or another irreversible dementia. Medications, an acute illness, or even a brain tumor may be causing the problem. Nevertheless, over the long run for the vast majority of persons with memory loss, it is important to monitor function carefully. Occasional forgetfulness, such as forgetting names, is not a major problem. Yet when the older adult begins to leave the stove on, repeatedly loses his car keys and cannot locate them, cannot keep up with usual personal business (such as banking) or cannot find his way home driving from a familiar place, then help (often from a social worker) is most important to maintain independent living as long as possible. Alzheimer's support groups are found in most communities and they can be of great assistance to people with Alzheimer's and their families. You can start with your state or local office on aging or a local chapter of the Alzheimer's Association.
In such situations, I will revert to the "family conference." In such a setting, all significant members of the family (spouse if living, children, siblings) meet alone first and agree on the significance of the problem and what needs to be done with the assistance of a social worker who is skilled in the day-to-day management of memory problems. A plan is devised. Then the family meets with the older adult and as a group virtually insists that the older adult seek help, if not for her sake, then for the sake of the family. Have an appointment already set up. Have at least two family members go with the older adult to the evaluation and make certain that the family (in the presence of the older adult) express to the clinician the problems that have been noticed. This approach works most of the time IF the family is in agreement and speaks as one voice.
You have hit the nail on the head. Caregiver stress and burnout are among the most important factors that lead to institutionalization or even maltreatment of older adults with Alzheimer's. There are a number of steps which must be taken. First, your mom needs to seek professional help herself. She may be experiencing a major depression, which can be treated with a combination of medications and psychotherapy (but of course this does not remove the stress). In addition, your mom needs a break. If it can be arranged, she should be able to get out of the house a couple of hours a day by herself (even if she only does shopping or other errands). She should also be able to get away for more extended periods over the weekends, such as a Saturday afternoon and perhaps an opportunity to attend a religious service on Sundays if she has been a regular attendee in the past. A major problem that creates stress for the caregiver is sleep deprivation. If your dad does not sleep well, a medication to improve his sleep might be useful. Keep the doors of the house locked so your dad cannot walk out during the night and your mother may be able to sleep with more security about his safety. Bottom line is that your mom needs help for whatever symptoms she is displaying AND she needs a break from the care of your dad.