If my younger sister has polyarteritis nodosa, what is likelihood that her older brother myself, will have it too, i have no symptoms at the moment?

Unlikely: This is a very rare disease to have. It most commonly is associated with untreated hepatitis B virus. It can also be idiopathic, meaning, no associated cause is found. With autoimmune diseases in general, there is some inherited component typically, but it does not mean that exact disease will be inherited by another family member or that a family will even develop an autoimmune disease.