What is the ruling of the Shari'ah concerning the money and situation of the sister's husband who invested his money in the stock market with his knowledge of it, what is the ruling on eating from his house, and what should his wife do about this?

The transaction between your brother-in-law and the man is not clear. If the goods are in exchange for his money, then it is a permissible salam (forward sale) contract, provided its conditions are met, and the man owns the money and invests it for himself. However, if the goods represent a fixed profit paid every three months, then it is impermissible and invalidates the transaction. Speculation in stock exchanges (Bursat) is not permissible unless Sharia (definition)-compliant regulations are observed. A transaction involving mixed (halal (definition) and haram (definition)) funds is permissible, and there is no harm in eating his food or benefiting from his wealth. It is advisable to counsel him to fear Allah and avoid forbidden and doubtful matters, for there is no good in the forbidden, and its outcome is ruin for its perpetrator.