That is an old entry, that I found while browsing related forums.
As I understand, your mom might have been on some medication/s for her ailments. Magnesium or any supplement/herb can react with medications. If one takes more magnesium than the body needs, it will end up cleared by the kidneys, through the bowel - in form of diarrhea -, and or will provoke muscle relaxation, not shaking. Presence of shaking tell me there has to be another reason for it. Shaking/mental clouding can occur when imbalance of sodium is present; which can be expected if the kidneys worked too hard to eliminate the magnesium. In aged people doses are usually lower, due to slower metabolism and lesser reserves of the body to deal with elimination.