There's a bunch of information up there in the stickies on the forum.
It's not "cureable" -- it's just something you have and you have to treat it, like anything else.
It's not the end of the world -- women worry about two main things with PCOS: fertility and weight gain. Then there are a bunch of other things that affect you when the hormones are out of whack, but those two are the biggest ones.
I don't know what treatment your sister is going to get, but please have her do her own research and to listen to her body more than the doctor when it comes to whether treatment is working or not.
The hard part about PCOS is that what works for one person doesn't necessarily work as well for another, so it's really a trial-and-error kind of disease. It also varies a lot by the type of doctor that you have, so please recommend that your sister work with a reproductive endocrinologist to treat her.
Also, the symptoms vary, it's really dependent on the type of PCOS that you may have (again, the stickies on the forum can give you more information on that).
The hardest part of PCOS, I think, is that it's still very misunderstood and a lot of other problems are associated with PCOS, so it can get confusing in terms of what you're actually dealing with. There are also side effects that can appear -- someone with PCOS is more likely to get diabetes, for example (because of the insulin resistance that can come from PCOS).
Good luck and please make sure you guys learn as much as possible about it and treat it. Don't let it get out of control!