What makes Sairy Wilson a heroic character even though she is physically weak?

The Joads and the Sairy Wilson decide to help each other on the journey by spreading out the load between their two cars so that both families will make it to California.She and her family aid the Joads when Grampa Joad has a stroke, and decides to continue with the Joads on the way to California, for the two families can help each other on the way. She falls ill at the first camp where the two families stay, and remains there with the rest of her family, facing the possibility of arrest for trespassing.