Does an oath take effect by refraining from entering the house of the wife's family, and what is the expiation for this oath?

If you swore not to enter your wife's parents' house and then entered, you have broken your oath and must offer expiation for it. The expiation is one of four options, to be chosen freely: feeding ten poor people, or clothing them, or freeing a slave. If you are unable to do these three, then fasting for three days. If entering your wife's parents' house is beneficial, then it is better to offer expiation for your oath and enter, as the Prophet, peace and blessings be upon him, said: "If anyone of you takes an oath and then sees something better than it, he should offer expiation for his oath and do that which is better."