How can a dispute and estrangement between siblings be resolved, stemming from the brother reminding his sister that he helped her buy her house, while he refuses to take back the money he paid?

Repentance from severing ties of kinship is obligatory because it is among the major sins, as Allah Almighty says: ﴿So would you, if you were to turn away, cause corruption in the land and sever your ties of kinship?﴾, and the Prophet, peace and blessings be upon him, said: "A severer of ties will not enter Paradise." Likewise, the brother must repent from reminding of his favors (mann), for it is one of the major sins that nullify good deeds, as Allah Almighty says: ﴿O you who have believed, do not invalidate your charities with reminders of your favor or injury﴾, and the Prophet, peace and blessings be upon him, said: "There are three whom Allah will not speak to on the Day of Resurrection..." and he mentioned among them: "the one who reminds others of his favors." If what the brother paid was a debt and its acceptance was refused, then it is an absolution. If it was a gift, then it is not permissible to revoke it. One should be patient with the harm from people, especially relatives, and initiate the greeting of peace, for the Prophet, peace and blessings be upon him, said: "And the better of them is the one who initiates the salutation."