my sister has been diagnosed with this what is it

PCOS is Polycystic Ovarian Syndrome.

It means that she has -- or has the potential to have --lots of little cysts on her ovaries. It can cause weight gain and is sometimes hard to diagnose. She is lucky that her doctor was able to diagnose it. It took my specialist a long time to diagnose mine. She can take a drug called Metformin that regulates her hormones and helps her lose some weight if need be.

Don't worry.