Alzheimer's is truly a cruel disease, with its tragic loss of memory and its long descent into incapacity and eventual death. But it becomes even crueler when someone takes advantage of a person's diminished ability and cheats them.
I am not a lawyer, of course -- but it may be that your only option is to seek the advice of an attorney who is knowledgeable about estate law. I wish you could avoid it, of course, because the Bible urges us to avoid conflict whenever possible (particularly in a family); it says, "Make every effort to live in peace with all men" (Hebrews 12:14).
But sometimes conflict can't be avoided, particularly when something has been done that is clearly wrong. When Paul -- a Roman citizen -- was beaten and imprisoned by the authorities in Philippi, he demanded an apology from them after they released him; he knew it was illegal for them to beat a Roman citizen without a trial (see Acts 16:22-40).
Pray for your sister, not only that she will admit the wrongness of what she has done, but that she will realize she is on a wrong road spiritually. Only Christ can satisfy the spiritual hunger in her heart, and only Christ can give her the lasting peace and joy she seeks.