I'm with Aprilis on this - I have taken Citlopram daily for about the last eight years, and they keep me balanced. Prozac reacted very badly on me, but as both AB'ers confirm, different anti-depressants suit different people.
It will takew two to three weeks for the meication to settle properly in your sister's system, so avise her to be patient. If she gets side-effects, and they last beyond this time, tell her to consult her GP who will either change the dose, or the prescription, but do tell her not to adust the dose herself, or to stop taking the tablets - very important.