It's a horrible shock hearing that someone so young has a cancer; however, womb cancer seems to be one of the more treatable cancers so please don't despair. Although I'm much older (62), and although I was diagnosed at a very early stage, I had a radical hysterectomy at the start of this year, have had 4 chemo sessions and 3 brachytherapy sessions. At all stages of the treatment, I found it much easier than I had expected.
Assuming it is womb cancer, your sister might like to join a facebook group which supports women with womb cancer -
http://www.facebook.com/WombCancerSupportUK - where there are women at various stages of diagnosis and treatment. It's very supportive and friendly. I'm a member there and have had some great help through some wobbly moments.
I really hope that your sister's treatment works very well for her - you are obviously going to be a great support for her (which is also very important) as she starts on a challenging journey.
M