From what you've said, although your father was terminally ill and close to the end of his life, your family members did not utilize physician-assisted suicide, which is legal under Oregon law if certain conditions are met but took it upon themselves to end his life. Their actions would be unlawful.
If what you want to do is initiate a homicide complaint
, you should contact the DA's Office in the county where this took place and share the information you have found out with them. You would be able to do this before an exhumation. In fact, if you could already substantiate enough of your suspicions, the DA's office might be able to take over the exhumation hurdle for you or shortcut some of the exhumation difficulties you might otherwise face. In any case, they would take your matter seriously and guide you through exactly what you'd need to do to go forward. You'd be dealing with them as a complainant, so there would be no legal fees involved. Go in person or call and make an appointment to speak to a homicide DA.
If you do have to arrange the exhumation yourself without the support of the DA's Office, I'd expect a lot of family opposition. If your sister and niece were caretakers for your father at the time he died, they are open to civil liability (a wrongful death suit) as well as criminal. You will need a good local civil attorney for that, and I suspect the issues will be protracted and expensive, both financially and psychologically.
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This is informational only and is NOT legal advice. There is no attorney-client relationship. You are advised to consult an attorney in your state for specific legal advice.