Should relatives of a person with paget's disease get tested?

Yes. Because paget's disease can be hereditary, some experts recommend that the brothers, sisters, and children of people with paget's disease talk to their doctor about having an alkaline phosphatase blood test every 2 to 3 years after age 40. Paget's disease is rare in people under 40.
Yes and no. There are a few familial disease in which paget's (of bone) is early and severe; if these are present, a geneticist will be called at once & family screened. Mild paget's of bone is extremely common in older folks. It's not clear that early detection / treatment before symptoms develop is of any value. Elevated alkaline phosphatase in the blood is nonspecific but an inexpensive screen.