Hi Debbie,
I think it would be normal for a very depressed person not to want to get out of bed while she tries to come to terms with her situation. You might try to get her a prescription for antidepressants if she's not on them already - they work well for some people. There may also be the option of getting some professional help for her depression. And if she's on the internet she may find the website
www.lchelp.org
a valuable source of support. Hopefully some others will post in with more suggestions for helping to get your sister back on her feet.
You can find various links to lung cancer information and support on my webpage
www.cancerwebsites.org/lung
I hope this helps a bit.
Regards, Ed E.