Everyone says high cholesterol is genetic. my parents and sister are 260-300 and mine tests in around 100. same diet. how is this possible?

It's complicated: Concurrent disease, diet and fitness habits contribute, of course. We're sorting out the genetics of cholesterol. Especially, a few "longevity" genes can make total cholesterol freakishly low as in your case; these tend to be autosomal recessives. Perhaps your parents are unknowing and unaffected carriers. For the same reason, blue eyes occasionally pop up among even the darkest-skinned nations.

Interesting: If you live with your family members then your diet is the same but if you have lived away from your parents and sister it is difficult to say you have the same diet. When based on diet, amount is also a factor. You have similar genetics to your family but not the exact same. You have gotten different genes from each parent than your sister and may not have hypertension at the same age.