Sounds like there are quite a few issues here. Sounds as though the deceased's brother is the closest relative that can be located. First step would be to have him step forward to administer the estate and try to locate daughter...no one can legally handle any of his assets (including the farm) until an administrator is named. Based on the complexity you need an attorney.
This is not legal advice nor intended to create an attorney-client relationship. The information provided here is informational in nature only. This attorney may not be licensed in the jurisdiction which you have a question about so the answer could be only general in nature. Visit Steve Zelinger's website: http://www.stevenzelinger.com/
You need to have an administrator appointed to administer the estate to have any legal authority to act. Frequently this involves a petition to the probate court and notice to other potential heirs. Contact a probate attorney in the county where the Uncle died and before the consultation prepare a family tree showing the the deceased uncle, his parents and their descendents. Remember if any of his siblings died but left children they may have an interest. Even if you are not sure of addresses, provide the last known address for each individual in question so that the attorney knows who will need to be notified
Adopted children and biological children are treated the same under Washington law. In the facts as you have presented them, the adopted child would be the rightful heir to an intestate estate. That being said, you do not have to be an heir to petition to probate an estate.
While you are searching for the adopted child, one of the nieces or nephews could hire an attorney to begin probating the estate. Once a personal representative is appointed, that person has the authority to take care of the farm's affairs until ownership can be transferred. This might require some money up front, but attorneys' fees are usually chargeable to the estate.
Hope this helps!
This posting is for informational purposes only. It is not legal advice, nor does it establish an attorney-client relationship. For more information, please visit www.justinelderlaw.com.