My parents and I are named on the real estate certificate, but my father uses the real estate as collateral, so my mother and I have to sign it?
You and your mother need to sign, but you should transfer the property to your father. At the time of mortgage, the guarantor is your father. If you and your mother sign at the same time, then the guarantor is responsible for both of you. Then once your father doesn't repay the loan, you and your mother will be indirectly identified as responsible persons, which means the responsibility will be borne by the three of you.