Unless your father's friend in incompetent, your only alternative is to try to continue to work with her to preserve the property - perhaps by buying it from her if she needs money. If she is competent, she has the right to change her will and leave everything she received from your father to someone other than your brother and yourself. Discuss the matter with an estate planning attorney in your area.
You haven't identified the other parties involved in this question so I cannot determine whether I may have a conflict in this matter. Should it turn out that I have an attorney-client relationship with any of the other parties, my response to this question will not prevent me from continuing to represent an existing client.
I am not sure how you promised your dad you would help his friend. Financially? Is she elderly and in need of other support? What happens if she sells the house - does she get to keep all of the proceeds? Is she poor and you and your brother doing well? Do you have a good relationship with her?
Lots of questions and many of them are more practical than legal. I suggest that you talk with a local elder law or estate planning attorney to offer you suggestions based on what is important to you.
You can find good attorneys on this website or at naela.org
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