Well, you could write a fanfiction where the events take place in a different time period. For example, if the original story is set in modern times, set it in the past or a future with advanced medical technology. You could also develop a story around the unspoken feelings of the secondary characters. What if the brother had a secret plan all along to save his sister in a more unorthodox way? That could be an interesting plot.
Well, 'My Sister's Keeper' is a really touching story. It shows how a family is centered around the sick sister Kate. Anna, the younger sister, has been donating things like blood, bone marrow etc. for Kate all her life. But as she grows older, she realizes she has her own life to live. The court case she files against her parents is a major part of the story. It makes everyone think about what is right and wrong in such a difficult situation where a family is torn between saving one child and respecting the rights of another.