Idaho Code § 15-3-407
In contested cases, petitioners who seek to establish intestacy have the burden of establishing prima facie proof of death, venue, and heirship. Proponents of a will have the burden of establishing prima facie proof of due execution in all cases, and, if they are also petitioners, prima facie proof of death and venue. Contestants of a will have the burden of establishing lack of testamentary intent or capacity, undue influence, fraud, duress, mistake or revocation. Parties have the ultimate burden of persuasion as to matters with respect to which they have the initial burden of proof. If a will is opposed by the petition for probate of a later will revoking the former, it shall be determined first whether the later will is entitled to probate, and if a will is opposed by a petition for a declaration of intestacy, it shall be determined first whether the will is entitled to probate.
Idaho Code § 15-3-407