Is the questioner considered to have severed ties with his sister after she and her husband refused to notarize the sale of an apartment and a piece of land that had been agreed upon, even though the questioner had paid the agreed-upon amounts?

Maintaining ties with one's sister is obligatory, and severing them is forbidden. Injustice from the husband or the sister does not totally permit severing ties with her. Rather, ties must be maintained to the extent that it does not cause harm. Maintaining ties is not limited to visiting or calling; it encompasses everything that is conventionally considered a bond, such as financial assistance, fulfilling needs, visiting, and exchanging greetings through correspondence. If you do what you can to maintain ties and your sister insists on estrangement, then you are not the one severing kin ties; she is. You remain upon great goodness by seeking the pleasure of Allah, as stated in the Hadith: "If you are as you say, then it is as if you are feeding them hot ashes, and you will continue to have a supporter from Allah against them as long as you persist in that."