I am so sorry your sister is going through this experience and that you have to share it. You are a sweet and caring person. I am not a medical expert, but I do know from experience that stopping a medication like Paxil can have serious side effects. Depending on her dosage, age and mental state it can be quite traumatic for your body to stop like that. I would reccomend that she goes back on it immediately and talks to her doc about why she wants to stop taking it.
It would be best, as itsokay suggested, that she taper off, not just stop. Meds IMHO are not evil if we need them. I have come off a couple (understatment!!) anti-deps and they were all challenges, but if you go slow and take the right advice it is ok. I don't mean to pry, and you don't need to answer, but why did she want to stop?
I hope she (and you sweet big sister) are feeling better soon.