"how do I continue to cover for her and not make it seem as if she's not as committed to her job? (we work together)".
Tell her that you will not cover for her anymore. I'm not suggesting that you SAY anything to anyone about your sister's health, but if someone makes a comment to you, say nothing. You shouldn't be put in that position. It's not fair to you nor is it helping her in the least.
I would IMMEDIATLY ask your sister how SHE would like you to handle her cancer at your common workplace.
While I totally understand that cancer is a difficult thing to cope with, I've never understood why people want to hide the fact that they have cancer. Cancer has nothing to do with one's character, it can (and does) happen to anyone. Many people find comfort in sharing their journeys with one another while others do not. There is no right or wrong way. When you have cancer I found that I really needed the support of others.
Good luck to your sister and to you.