What should the sister do after learning that her husband's father, who provides for them, acquired his money through unlawful means, and what is the ruling on living in the apartment he built for them with this money?

It is not permissible to definitively state that a man's wealth is unlawful before confirming it. If the son confirms that all of his father's wealth is unlawful, then it is forbidden for him to take from it. However, if most of the wealth is unlawful, the Shafi'is disliked it, and some of them prohibited it. The son must earn a living to spend on himself and his family if his father's wealth is dubious. If he is unable or poor, then his expenses are on his father, and he may take from it, with the sin falling upon the father who earned the unlawful money. In this case, the son is like a poor stranger. As for residing in the apartment, there is no objection, but it is preferable to reside elsewhere if that is available.