This can be a problem indeed. There are a couple of things that need to be addressed - why is she doing it and how to keep her from doing it. First, is she soiled when she does it? Although she has Alzheimers, she is aware of certain feelings. She may be uncomfortably wet and not like the feeling -even momentarily- of being wet. She may also feel that she has the urge to urinate or move her bowels. Those sensations all lead her to "investigate" the situation. A toileting/diapering schedule helps. Keep her dry and comfortable. Additionally, there may be an underlying physical issue., eg, time, bladder or kidney infection, constipation, etc. Make sure all of those possibilities have been ruled out. If she is using tape or velcro closure diapers, find a better-fitting brand. Often the brief or diaper is too large. Tighter fit gives little room for wandering hands. Another option although it is definitely not the preferred for various reasons, is to use a pull-up or even large pair of underwear over the diaper. It creates an extra barrier to access. Tucking in tee-shirts or jerseys into her pants also impedes access. Of course, redirecting your mother's behavior also is key. Give her something to hold in her hands all the time- a doll, a balled-up pair of socks, or even better to solve both the wandering hands and need for sucking, offer her a mildly frozen/chilled washcloth. (Not frozen solid or iced cold though- we don't want her hands or mouth to get frostbite.)
Hope this helps. Now is the time to get creative. This stage too shall pass. Hang in there.