Would a husband be sinful if he were to gift his only house to his first wife to appease her, and he has no other house to give to his second wife, and is he required to retract his gift, or is the house forbidden to him?

A husband must be just between his wives in division (of time) and expenditure. It is commendable – but not obligatory – to equalize between them in gifts and presents. There is no harm in a husband gifting his house to his first wife without the second. However, some scholars do not validate a man's gift of his dwelling house to his wife if he continues to live in it with her until he dies.