@Misscoco - I think that your sister should go for a checkup with an ear, nose and throat doctor.
It sounds a lot like the condition my friend at work has. Sometimes she has to make presentations at work. Her voice sounds, tense, tired and strained. Afterwards, she says her throat and neck hurt.
Anyway, she finally went to the doctor and he said she probably has something called. muscle tension dysphonia. It may be a medical condition and it may be muscle tension. This condition should be evaluated by a speech therapist.
The treatment the doctor suggested was voice therapy by a specialist, to relax muscles in the throat and neck. Voice therapy has worked pretty well in helping the throat to relax.