What state did the death occur in?
What is the estimated value of this estate?
How was the "relative" (who took her to the attorney's office) related to the aunt?
Please explain what you mean when you say that you are being asked to "sign off" in this matter--are you being asked to disclaim or deny your inheritance or are you being asked to do something else? Do not sign off on, or agree to anything that you do not understand.
A person who has been officially diagnosed with dementia, in most states, is presumed to not have the mental capacity to compose legal documents, so the will is most likely invalid to begin with, and the "relative" who helped get the will written probably knows or suspects that you all will figure out the illegality of what he/she is trying to do.
Bottom line is that one (or all) of the nieces/nephews need to hire the services of their own attorney to let the attorney know that this will was written under duress and by a person with diminished mental capacity, so that the attorney can review the law to see if this will can be thrown out on a technicality, in which case the estate might be divided amongst the nieces and nephews anyway on the basis of your state's intestate probate law (if nieces and nephews would qualify). You also need to ask the attorney if you all need to contest the will, which might be somewhat expensive in legal fees to pursue such a case. I'm hoping that the will is going to be thrown out on a technicality and that you won't have to contest.