Can you tell me the story from the Sai Satcharitra about the doctor's nephew who was cured by Udi?

Certainly. Chapter 34 of the Shri Sai Satcharitra narrates the story of a doctor from Malegaon whose young nephew suffered from a severe bone-cyst, or Hadyavran. Despite the uncle being a doctor and consulting numerous physician friends and expert surgeons, all medical treatments, both native and foreign, failed. After exhausting all options, the desperate family heard of Sai Baba of Shirdi. They traveled to meet him, and upon hearing their plea, the compassionate Baba gave them Udi and instructed them to apply it to the wound, reassuring them that those who seek shelter in the Masjid shall never face downfall.