As you mentioned that the X-ray is normal, I believe that she has been examined by a doctor and any other injury
has been ruled out.
The dark patches below the eyes are due to the collection of blood in the layer below the skin (bruise
). It shall take a few days, usually 5-7 days, for the blood to be absorbed and nothing needs to be done. In case of recent injury ice packs are advised to reduce the accumulation of the blood.
Wishing your sister an early recovery.