Hi KP1999, I know where you're coming from. When my younger sister was diagnosed it was heartbreaking. Like coll0042 said, the mundane things like shopping, housework are important and sometimes just being there with her, in your pjs and watching Netflix is just as supportive. 
Your sister may not take everything in that's being said by the doctors either so maybe write down any questions she may have so she can take them to her appointments. My sister used to forget what she wanted to ask and remember when we left. I started a notebook and wrote everything down. I still have it now, 6yrs on!

It doesn't matter how trivial the questions seem, ask away, the doctors don't mind. 

When she's going through chemo, she may not want to eat. Try pre-making some soups or her favourite foods and have some biscuits handy. A cup of tea and some breakfast biscuits maybe all she wants. Ginger ones are good for any sicky feeling and do are the complain shakes. It might only be a shake but they have the nutrients. 
Radiotherapy doesn't take long in itself, around 20mins but being daily it can take its toll on your sister and whoever takes her.
You'll get a schedule of appointments, try and spread them out between people if you can, you'll need to keep your strength up to support your sister so accept any help if it's offered. 
It will be a bit of a rollercoaster and there maybe times you also need some support. 
I called Macmillon just to talk to someone outside of the family, they were great, this chat forum is good too. 
Feel free to ask any questions or vent if you need to. 
Take care, wishing you all the best xx