There's no way for any of us here to give your sister advice about whether or not the "deal" is a good one or not without a face-to-face consultation and a thorough review of the facts of the case.
It's like asking an on-line medical site whether or not you should have surgery.
If your sister doesn't think she's guilty, then she should NOT plead guilty. If the public defender is just telling her to plead out, something's wrong. Your sister may want to do what it takes to hire a good criminal defense attorney on her own. Her freedom and criminal record are at stake.
You don't say what area your sister is in, but a search on Avvo.com might be a place to start.
I agree with counselor Dane. I would never plead guilty to something I felt I was not guilty of. A criminal record will follow you for the rest of your life. If the PD is suggesting a course your sister disagrees with, perhaps it is time to discuss the situation with her own attorney. It would be money well spent if it helps avoid a conviction.
Whether someone should accept a plea depends on the circumstances. I don't think something is necessarily wrong if an attorney -- public defender or privately hired -- has carefully reviewed all of the evidence and weighed the facts, and tells a client that they are likely to do far worse if they take the case to trial.
Keep in mind that people sometimes are embarrassed to share all the facts with family members, and the attorney probably knows more about the case than you do.
You are also personally involved in your sister's matter, so it's understandable that your emotions would affect the way you view the case. A good criminal defense attorney can look at the case from an objective standpoint and see how a judge or jury would look at the evidence. Sometimes even innocent people do things that wind up making them look guilty.
The decision is ultimately up to your sister, who has to decide whether the sure thing of a plea bargain is better than taking the risk of trial.