Based on details your daughter has chronic headache.MRI scan revealed sinusitis
Most common cause of headache at this age is migraine headache
.Mostly she may be suffering from mixed headache ( migraine headache + sinusitis).
Character of headache and aggravation factors helps to know the cause of headache.Consult Neurologist
and ENT surgeon for mixed headache.
Short course of antibiotics of 1 week duration ,along with decongestants will help to relieve sinus headache
.If Headache still persists then one need to start migraine prophylaxis drugs.