You did not say if your mother has yet passed away. If she has not passed away, the will cannot be contested because it has not yet taken affect. The proper thing to do is raise an issue of your mother's competency in a guardianship proceeding.
If you mother has died, the time to challenge the will is dependent upon whether or not the will has been probated. Speak with an attorney in Gettysburg who is familar with will contests.
To establish a case of undue influence, you must establish the following elements: (1) A confidential relationship between the proponent of the will (your sister), (2) The propent of the will received a substantial benefit under the will, and (3) the testator suffered from a weakened intellect.
In other words, this is not just a simple "form" You need high-quality advice from an attorney.