The will is invalid. Under Indiana law in the 1860's Theresa, as a married woman, has no property that was her's to bequeath. Later on, Zacharias has a change of heart and refused to pay. The three would-be heirs sue. Zacharias defends by claiming there is no consideration for the promise.
Consideration doctrine focuses on such promises.
It generally regards them as supported by consideration. Consideration doctrine tends to regard promises that are not part of such an exchange as lacking consideration. A commitment to pay 10 days rent. In this sense, their legal obligation under the contract is just to pay ten days rent. She could not have been seeking that minimal commitment in exchange for her promise.