Your chances are greater than if there were no Alzheimer's in the family. However, your mother's age is a big factor. If she was diagnosed when she was young, there is likely a gene that will put you at greater risk. If she was quite elderly, say 75 or 80, well half of us will have Alzheimer's if we reach 85, so that doesn't put you at so much of a risk (any more than all of us).
There are still many studies being done on percentages and risk, but you can exercise, avoid a high fat diet, take essential fatty acids and keep your brain active. These steps have been shown, in some studies, to help. They can't hurt.
Keep in touch. It's great to hear from you.