What should a married man do who treats his wife kindly, while her family (her brother and his family) treat his sister poorly?

You must continue to treat your wife kindly, following the example of the Quran and Sunnah, and do not punish her for someone else's sin. As for your sister's husband, you should advise him, reminding him of the obligation to treat his wife well, and to instruct his family to stop mistreating her. If he does not respond, you can seek help from righteous and virtuous people to influence him. If he continues to harm your sister, she has the right to seek divorce, khul', or to refer her case to a judge. It is preferable to resolve family disputes without resorting to courts unless absolutely necessary.