Neb. Rev. Stat. § 30-2720

(a) By a writing signed by all parties, the parties may designate as agent of all parties on an account a person other than a party. (b) Unless the terms of an agency designation provide that the authority of the agent terminates on disability or incapacity of a party, the agent's authority survives disability and incapacity. The agent may act for a disabled or incapacitated party until the authority of the agent is terminated. (c) Death of the sole party or last surviving party terminates the authority of an agent.