First, email the Kidney Cancer Association at office@curekidneycancer.org, and ask for the free Incidental Finding book (include your name, postal address and telephone number (in case they have a question). They will send it anywhere in the world.
You may also want to join the KIDNEY-ONC email support list specific to kidney cancer. See
http://cancerguide.org/kofaq/ .
Both of these will give you information on treatments that might be available for your sister. If it is true that your sister's cancer is too far advanced for treatment, whether or not you tell her about her illness is something you probably know already. I am sure that she realizes that she has a serious medical problem, even if she doesn't know what it is. Personally, I would want to know, but I know not everyone feels that way. If you do not tell her, you will be setting up an emotional wall between you, and your will not be able to ask her how you can help her do the things she might want to do while she is alive.