The "bad" sister forged my fathers signature on the MOST form. What are our options?

Typically when forming legal agreements, it is best to have 2 people, a primary and a backup, if possible. I think often these forms have to be witnessed and notarized by third parties to complete correctly, hopefully. With health choices, DNR is one of the several options and people in their late 80s either can have had so many surgeries or have heart-related issues, that they think sensibly about DNR is best choice. That is just process statement.

In regards to suspicion of forgery of documents, one would think those should be questioned/reviewed again and would require an attorney to review those legal documents as well as financial advisers to understand scope of coverage financially and health-wise. People take advantage of people all the time, and methods are often questionable in how they obtain information or get an aging parent to agree to a so-called deal. But these titles of POA, DPOA, etc are legal issues and should be reviewed again.