I think this depends on a couple of things. I want to be sure I understand your facts. When you say your sister "has alternate DPA," do you mean that you are named as the agent and if you cannot act, then your sister is named as alternate agent? OR do you mean that there is a second DPA form that names your sister as the agent? If there are two POA forms, then you may well be stuck. If there is only one form, then more information is needed. Presumably, in that case, your sister would have had to present something indicating that you were unable to act. If that was done fraudulently, then you may have a claim against your sister. It SOUNDS like the auction has already taken place. If that is true, then it is unlikely you will be able to unwind the auction.
I would suggest that you contact a probate lawyer, right away, to determine whether or not you can (and should) pursue an action against your sister. You also need to clarify what rights your sister has, if any, going forward, with respect to your parent's assets. When you say "estate," I assume you do not mean that term in the sense that your parent has died. If you DO, then it is likely that your sister is not acting under a POA at all, and rather is acting as executor or administrator for the estate. That is a completely different scenario.
***Please be sure to mark if you find the answer "helpful" or a "best" answer. Thank you! I hope this helps. ***************************************** LEGAL DISCLAIMER I am licensed to practice law in the State of Michigan and have offices in Wayne and Ingham Counties. My practice is focused in the areas of estate planning and probate administration. I am ethically required to state that the above answer does not create an attorney/client relationship. These responses should be considered general legal education and are intended to provide general information about the question asked. Frequently, the question does not include important facts that, if known, could significantly change the answer. Information provided on this site should not be used as a substitute for competent legal advice from a licensed attorney that practices in your state. The law changes frequently and varies from state to state. If I refer to your state's laws, you should not rely on what I say; I just did a quick Internet search and found something that looked relevant that I hoped you would find helpful. You should verify and confirm any information provided with an attorney licensed in your state. I hope you our answer helpful!Ask a similar question