Abandoning the performance of Salah is Kufr (disbelief). Based on that, the Sawm of someone who neglects Salah is Batil (null and void). If your brother-in-law is as you say, advise him, and seek the assistance of the righteous people you know in advising and guiding him. If he returns to the right path, all praise be to Allah, if not, ask him to leave your house and do not live with him or offer him hospitality, as he does not deserve such hospitality.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.