My sister got power attorney and cut me and my brother out. Is there anything we can do about the matter?

Family dynamics can be challenging! Consult an elder care attorney to discuss your concerns. As POA, your sister is empowered to make decisions for your mother which include placing her in assisted living, selling her home, and managing her finances unless otherwise prohibited by state law. She did not need your permission to sell the home. And she chose not to advise you or your brother that the sale was imminent or offer any keepsakes. The questions you want to be answered include: (1) Whether or not your mother was competent when she gave your sister POA, and (2) Whether your sister is managing the proceeds from the sale of the home and its contents to your mother's benefit, and (3) Will there be any funds available for beneficiaries after mom is gone. An attorney can advise you whether or not you are entitled to an accounting of your mom's finances while she's still living and/or when she is deceased. My guess is your sister has commingled funds or put the sale proceeds solely in her name which further complicates the issue.

A POA cannot change a will. A will doesn't kick in until someone is DEAD. A POA would allow, yes, your Mom to be placed if this is for her good, her safety. It would allow the daughter, your sister, to sell assets to pay for the care of your Mother. A will is only as good as the assets it still holds at death. They may be spent already, every single cent, in which case no one will get an inheritance. An elder's assets should be for their care while they are living.

Is your mother still living? If so, perhaps the home was sold to pay for AL. Paying for AL is very expensive so perhaps your sister isn't trying to cut you and your brother out, perhaps she's trying to provide for your mother's care. Would your sister provide an accounting to you and your brother so that you can see the sale of the house as well as payments made from that money?

The POA is supposed to be for your mom's own care in AL/SNF etc.

After she passes, the will is in place. If the will still says the estate will be split into thirds, that is the way it will go.