Welcome to HealthcareMagic forum .
Your sister does seem to have most probably a upper respiratory tract
infection causing pharyngitis
( sometimes may appear as white spots
on red background ).
Most common cold
or viral infections should resolve or decrease in 2 weeks .Since your sister has persistent infection she most probably needs a course of antibiotics which can be prescribed by your physician after confirming diagnosis. Any one of , a Physician or a chest physician ( pulmonologist ) or an Ear nose throat surgeon can be consulted & they all can handle such cases well. Since she has not been to the doctor it's advisable to visit your family physician first .
Hope I have answered your query .
Wishing you and your family good health.