What should the two brothers do about their sister's refusal to sell them the two apartments gifted to them by their father, and should they reject the gift for it to become an inheritance eligible for sale?

If a father gifts an apartment to each of his sons, and each son takes possession of what was gifted to him, then the gift is valid and enforceable, and the donee has the right to dispose of it. A condition that contradicts the essence of the gift, such as prohibiting its sale or re-gifting, is void, while the gift itself remains valid. So, if each son takes possession of his apartment, he has the right to dispose of it by sale or otherwise, and the father's condition of not selling is not considered. If one of the sons sells his apartment to an outsider, the neighbors have the right of pre-emption (shufa'a). The sons should avoid anything that displeases their father during his lifetime and strive for reconciliation with their sister to avoid discord.