It is most likely that Sandy Allen had Marfan Syndrome. I only know this because I work at a non-profit organization that provides recreational therapy (read: skiing) for people with disabilities (physical as well as cognitive) and we happen to have a student that we serve, a 14-year-old girl, who, among other things, also has Marfan Sydrome.
People with Marfan's syndrome are "most often unusually tall and slender, with particularly long arms, legs, and fingers in comparison with the rest of their body." Also, the tend to have an indented or protruding breastbone (that eventually looks like a third breast that sticks out farther than the typical two) and heart problems. Technically what happens is that when the Sydrome "kicks in," the person's body never stops growing. For the aforementioned student, it hasn't "kicked in" yet.
"It is estimated that about one in 10,000 people has Marfan's syndrome. This means that it affects over 20,000 people in the U.S. alone. While there is no cure, there are effective treatments, and most people with the disorder live full active lives."
reference:
http://www.healthscout.com/ency/68/651/main.html