Its unfortunate that your sister is suffering with blood cancer. As per history she might having cml, and receiving imatinib for treatment. Imatinib is known to cause musculoskeletal pain. Some time simple analgesic is helpful in getting relief of pain but many a times is due to more complex process that involve calcium and phosphate handling of body.
I strongly advise you to consult your primary oncologist/hematologist, he might ask for some investigation, and may try to modify dose (depend on recent bcr-abl copy number).
Hope this will help you.