1. The owner of this house must be the person who signed the sales contract with you. If this house doesn't belong to this man, you can't sign it, only with his sister.
2. When signing the contract with his sister, be sure to specify when the transfer will be made. Before the transfer, you can only pay the down payment, not the full amount. All payments must be made at the same time as the transfer.
3. It is certainly better to increase the guarantee and notarization of the intermediary, but the principle that must be paid attention to is that the contract can only be signed with the head of the household (that is, the person on the real estate license), and the full payment and transfer are carried out at the same time.