Undue influence can happen when someone who benefits from a will encourages the will's testator to create the will for the influencer's benefit. Merely encouraging someone to make a will does not create undue influence.
A common example is one child convincing his parents to leave him more in the will than his siblings. The siblings will be upset and may decide to challenge the will.
If the court does not believe there was a valid reason for the different inheritances, then the court will invalidate it on the grounds of undue influence.
An estate planning attorney can advise you in creating a will that fits your unique circumstances and follows your wishes.
Reference: My Prime Time News (Jan. 18, 2017) "Undue Influence."