Is it permissible to sell a disputed share in an apartment to a brother, given that the sister to whom the installments were paid has gone back on her promise regarding the agreed-upon percentage?

If you paid installments for your sister's apartment without an agreement, and then she told you that you own 7% of it, this is a valid sale that makes you an owner of this share, and she cannot revoke it. If you agreed to sell your share to her for 4,0 dollars in 2018, then you have sold your share, and the amount has become a debt owed by her. However, if you did not agree, or if her statement was merely an informing of the value of your share, then your ownership of the share remains valid, and you have the right to sell it to your brother. Nevertheless, your sister has the right of pre-emption (shufa'a) and can take the share for the price you agreed upon with your brother; this is based on the hadith narrated by Jabir (may Allah be pleased with him): "The Messenger of Allah (peace be upon him) ordained pre-emption in every undivided property..."