My aunt with full Alzheimer's signed a new will giving everything to a neighbor and died 4 weeks later. Can this be contested?

Her neighbors looked in on her and eventually got control of the bank account. She was diagnosed 4 months before signing a brand new will leaving everything to the neighbors. At the funeral, the neighbors professed no knowledge of a will or who the executers would be, Now they want my mother, her sister, to waive any claims to the assets.
Her previous will left everything to a charity. We suspect fraud. What can we do? - Is this your question? Add additional information