This is a complicated enough situation that you will need to consider hiring a local attorney.
That said, several things are important. First of all, power of attorney ceases at the time of death. If your parent is deceased, the power of attorney means nothing.
I assume there was a will? Who is named in the will as personal representative/executor? If there is no will, and no surviving spouse, the siblings have equal authority to begin probate. Someone needs to do that. The process varies by state, and again you should hire a local attorney to help you and begin the process NOW so that you can get past the impasse you have reached.
In MD the authority granted in a POA cease once the named agent knows that the Grantor has died. To sell the house and estate needs to be opened and a Personal Representative PR (executor) needs to appointed by the Orphans' Court through the Register of Wills
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