What is the ruling of the Sharia regarding a husband's insistence that his wife deal with and host his ill-mannered sister in their home, despite the husband's awareness of his sister's bad behavior and her causing problems for his wife, especially when the husband spends lavishly on his sister from the family's money?

A wife must obey her husband in what is right and good, and this includes treating his family well. However, she is not obligated to obey her husband by visiting or accompanying his sister if doing so would harm the wife's religious commitment, because "there is no obedience to a creature in disobedience to the Creator." You should explain to your husband the harms of associating with his sister, if what you mentioned about her is true, and remind him of his duty towards her by enjoining what is right and forbidding what is wrong. You should also consistently advise the husband's sister, remind her of Allah, and supplicate for her guidance.