How come i have pseudotumor cerebri, but nobody else in the family has it?

It isn't hereditary: Not every thing is hereditary. And sometimes this is a complication from another illness.

Pseudotumor cerebri: Pseudotumor cerebri is usually sporadic and there is rarely a hereditary component. Because it is most frequently associated with obesity, and obesity often occurs in multiple generations of the same family, pseudotumor cerebri can appear to be hereditary.