There's more implications than you mention here if your sister co-signs for the home. First, she needs to establish a credit score. She can possibly do that with non-traditional means such as cell phone bills, rental payment receipts and things like that. And it's in her best interest to establish credit history. Most lenders can help her, or she can contact Blue Water Credit and ask them to help her.
But co-signing on a mortgage for you is not normally in her best interest. She takes on 30 years worth of obligation for a debt that she does not get the benefit of enjoying. 30 years is a long time,and should you at any time default on that loan, it would reflect negatively on her. In fact, she would be expected to pay it back should you not be able to pay. Should she EVER want to get a home of her own, she would be showing 100% obligation on your mortgage and have difficulty getting one for her own.
You might want to consider whether there is another alternative, or discussing the full implications of her co-signing so that it's clearly understood.