My father is deceased and my mother has advancing Alzheimer's Disease. They had both always maintained that upon their deaths the property was to be equitably divided between my sister, my brother and myself. My sister had been living in a mobile home behind our mother and helping her with bills, etc for several years. My sister passed away in Jan 2012 and my brother and his wife took over the assistance. At that time, the agreement between the two of us was that my husband and I would complete a previous job commitment, then move back to take over her care (with the understanding that I would come back alone at any time if needed). We arrived for the Thanksgiving holidays to discover that my brother has dug a foundation and is beginning to build a house in the center of the property. When asked about it he became very combative.

My question is, If allowed to complete and move into the house, will this give him some sort of adverse claim to the entire property? I know that my mother had not previously deeded anything to him, and she is by no means competent to do so now.

I apologize for the length of this question

Does anyone have mom's power of attorney?

Unsure at this time, a lot of 'underhanded' things seem to be going on. How would I find out?

Also, does she have a will, and do you know where it is, and the lawyer who helped with it? And what state are you in?

I don't see an adverse claim unless he got her to sign something you don't know about. But there may be a lot of wrangling about the value of a year of care so far, and costs he's putting into a new structure. You could both spend a ton on lawyers. I would consult one now and see if you can work something out with the least expense. (I have to admit I feel sorry for those who lived in the trailer while taking care of mom... but that's aside from the law.)

Edit: what underhanded things

No will, they thought things would be divided peacefully. We are in Tennessee. The trailer was my sister's, it was put there when she first got married and immediately became pregnant with twins. Our mom and dad cared for the twins while she worked. She and her husband divorced several years before her death. The twins are now 18.

Just one of the snags that can happen: if your dad's estate wasn't probated and mom wasn't on the deed, nothing can be done with the property until it is, and then her's can be probated. Another is the trailer, depending on whether it's considered a vehicle or structure, and whether your sister had a will. The 2 18 year olds may decide it's theirs.
You can look into everything including POA (there are several kinds for different powers, such as access to funds). Ask her bank, ask the probate court, ask the deed office. Or hire a lawyer to look into all the above because it isn't going to be easy long distance. If your brother doesn't have POA and you get it, he's just going to be so angry that I'll bet that this will be extra costly for both of you after mom dies. Regarding underhanded things, I assume they involve spending her money on the new building and other things. The building will just be part of the estate for both of you. Personal expenses already spent will be tough to fight, maybe impossible. Your brother apparently doesn't want to leave the property now, even if you are about ready to take over mom's care. Of course you can force a sale of the place after she is gone. The sooner you get involved the better.
A reasonable solution would be for him to buy you out after she dies, if he can afford to.

Yes, and while you may think mom is not capable of signing, the court does not know this. So mom may have already signed it all over to the other family member, leaving you nothing.