If most of your family has alzheimer's disease, is it possible to not have gene?

Yes. The highest risk for alzheimer occurs with autosomal dominant inheritance (rare, because most alz is multifactorial with low risk for relatives). Ad alzheimer affects individuals in several generations, often with early age of onset. If your parent has ad alzheimer, your risk is 50% to get the abnormal gene and 50% to get the normal copy--50% chance you won't inherit the disease.
Yes. Genetic alzheimer's disease tends to hit early (mid fifties). If your family members are getting it in their eighties, then they're just like everyone else. Only about 3 to 5% of alzheimer's disease is of the hereditary variety. Most of it is just wear & tear on the brain.