Height of a person is determined by a number of factors like heredity, any underlying endocrinal diseases or deficiency, poor diet, recurrent illness etc.,
It is important to identify the cause in your sister. Also if your parents are on the shorter side, then chances of your sister growing very tall is slim.
You have mentioned the 3 accidents that occurred to your sister, but let me assure you that these accidents have nothing to do with the short height.
Take her to your family physician who might later suggest an endocrinologist
opinion, for finding out the cause and hence the solution for it. Also if she has started with her menses, the chances of height gain becomes less( as a girl's growth slows down after the periods start. The growth occurs for 1-2 yrs after onset of menses).
Do not worry !!!!!