What is the ruling on lying to the husband to recover an amount stolen by a relative?

It is not obligatory to pay what your brother stole to your mother-in-law, but there is no harm in donating it. If you borrow for this purpose, you are not required to inform your husband, but do not lie to him. You may use allusions and circumlocution. Your brother should be advised, reminded of God, and urged to repent.