Can you tell me the story from the Satcharitra about the doctor's nephew who had a bone-cyst?

Certainly. Chapter 34 of the Shri Sai Satcharitra narrates the story of a degree-holding doctor from Malegaon whose nephew suffered from a painful bone-cyst, or Hadyavran. Despite the doctor himself being a physician and enlisting the help of expert friends and surgeons, the ailment could not be cured. They tried all native and foreign treatments, including surgery, but nothing worked. At their wit's end, the family heard about Sai Baba of Shirdi. As a last resort, the boy's parents traveled to Shirdi, pleaded with Baba for help, and described their son's immense suffering. Sai Baba, being the embodiment of compassion, reassured them, told them to be worry-free, and gave them Udi to smear on the wound, promising a cure where all medicine had failed.