What can I do when builder do not registered the flat after taking full payment?

The property was registered to the flat promoter in November 2022. The sale deed indicated that the vendors (which include My wife, sister, mother-in-law, and her elder brother) acknowledged receiving the full amount from the vender, but the payment was made only to the elder brother's account without My wife's authorization. My Wife did not receive any money from the builder and did not give any authorization for the sale amount to be remitted to her brother's account. She did not sign the Development Agreement cum General Power of Attorney sent by her brother but was verbally assured a flat by him before the registration. After the registration, the brother is refusing to provide the promised flat and has mentioned offering only a small amount which he intends to pay after six months in 2024. The builder has already sold three flats out of the five constructed floors and aims to complete construction by May 2024. Pls suggest how to proceed further.