1) Your father's handwritten stipulation may very well not be enforceable--wills may be modified or changed, but only if done and witnessed in the proper way, and usually, simply writing in a handwritten addition, is not good enough. If you haven't already, you might bring the will to a trusts and estates attorney to see if this stipulation is in fact enforceable.2) If you have already bought the land from your sister or already have an agreement/contract with her to buy it, she cannot now add another term or condition; that is, if the deal is agreed to or consummated, it has to go through on the terms that had already been negotiated. In that case, your sister can certanly request you don't give or sell the land to the caretaker, but she can't enforce her wishes. A stipulation added unilaterally after the fact by one party is generally not enforceable.3) If you have not yet already bought the land and also have not yet executed an agreement with your sister, she may make this a condition of sale--she could write into the contract, for example, that in buying the land, you agree  to not convey any, in any fashion, to the caretaker. Your choice then would be to agree to the condition and buy the land; or not agree and walk away from the sale.