I read about a girl who was so weak she looked like twigs. How did Baba's Udi and blessings cure her?

You are referring to the story of Khimji Lalji Joshi's daughter, detailed in Chapter 33. Her illness was indeed very serious, and she was so weak that her hands and feet were described as being like twigs. Her mother first obtained Sai Baba's Udi, which provided some immediate relief. Seeing this, the family took the daughter to Shirdi. As she was too weak to walk, a man had to carry her to Baba. Shri Sai applied Udi to her and blessed her. Remarkably, within two to four days, she regained enough strength to start walking on her own. When she still had some trouble getting up, Baba gave her a fruit to eat, which completely removed that difficulty as well.