For women who do not ovulate on their own, Clomid is the "mildest" drug prescribed. However, I think a lot of docs prescribe too high a starting dose and also ignore any underlying issues. If your sister is having anovulatory cycles, she might have PCOS or another endocrine disorder, and that should be treated before trying Clomid. Clomid works for some women (it worked in the first cycle for me) but it has the unpleasant side effect of drying up CM for many women (this is uncomfortable and causes sperm to die before reaching the target), and also causes hot flashes and other nuisance s/e. As for the Provera, does she take that after a certain amount of days or what? If Clomid does anything for her, she shouldn't need Provera...she should be pregnant or get her period following ovulation. If she is told to take it on a certain cycle day, it may be actually preventing her from ovulating. When I took Clomid, I didn't ovulate until cycle day 21.
There are real risks with Clomid too. Many docs follow a "six cycles" rule, meaning they won't prescribe more than six or so cycles, because more than that may contribute to raised cancer risk. Another risk is ovarian hyperstimulation. Is your sister being monitored at all, especially if her dose of Clomid is high?
As for it being too soon, I really think that's something that can't be quantified. It may be better to tackle fertility issues now rather than put them off. But do suggest she sees an endocrinologist, preferably a reproductive endo, because she may have issues that would be treated better by an endo than an OB/GYN.