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Are there any known Addison's sufferers in the Kennedy political family?
Yes. His younger sister, Eunice Mary Kennedy.
JFK was formally diagnosed as having Addison's disease in September 1947, aged 30. He also suffered from hypothyroidism. His Wikipedia page notes that:
The presence of two endocrine diseases raises the possibility that Kennedy had autoimmune polyendocrine syndrome type 2 (APS 2).
About half of the people who suffer with APS-2 also have relatives who are affected by an autoimmune disease. [source: Barbara A. Majeroni & Parag Patel: Autoimmune Polyglandular Syndrome, Type II, Am Fam Physician, 2007].
Although the inheritance pattern in APS-2 appears to be determined by multiple genes, given that link, it is perhaps unsurprising that a 2009 review of Kennedy's case stated:
It is public knowledge that Kennedy's younger sister, Eunice, had Addison disease and that his son, John F. Kennedy, Jr., had Graves disease.
If you are interested, the story of, and background to, JFK's diagnosis with Addison's disease is covered in Joan & Clay Blair's book, The search for JFK, pp 560-