You say there has been a diagnosis of AD. This presupposes that there have been symptoms already and that there have been tests and an outcome.
Frequently a diagnosis of AD or any dementia will be made only once other possibilities have been ruled out
It is difficult to try and make an interpretation with so little information and the answer can only be.... could be, might not be.
There are different dementias though frequently the one quoted is Alzheimer's. Sometimes strokes are involved with dementia, sometimes strokes happen anyway. The development of dementias can lead to a loss of mobility. That may be loss of physical capability, or may be due to mental uncertainty and fear [of falling, getting lost, etc].
Why don't you start to gather as much information as you reasonable can about her current and recent state. Only when you have a baseline of information can you observe whether things have gone up, or down. Things will go up and down anyway, as the body works on cyclical patterns, so you need to observe over a lengthy period of time.
Such observations can help doctors greatly, as they generally have only the short period of a consultation to work on, unless the patient has been taken for an extended assessment period.
Of course your ability to do this is dependent on the frequency with which you see your mother-in-law, anyway!
Do it anyway. Sometimes it is beneficial not to see the patient daily as changes in capability are more masked because changes may be quite small individually, but more noticeable when they have happened one after another. Less frequent visits mean that you will note changes more obviously.
There are precious few easy answers! Best of luck.