Wis. Stat. § 853.13: Contracts.

(1) A contract to make a will or devise, not to revoke a will or devise or to die intestate may be established only by any of the following:

(a) Provisions of a will stating the material provisions of the contract.

(b) An express reference in a will to a contract and extrinsic evidence proving the terms of the contract.

(c) A valid written contract, including a marital property agreement under s. 766.58 (3) (e).

(d) Clear and convincing extrinsic evidence.

(2) The execution of a joint will or mutual wills does not create a presumption of a contract not to revoke the will or wills.