Her husband's brother rarely prays, so how should she deal with him?

Q: The sister who used the code name Umm Abdullah from Riyadh says in one of her questions: My husband has a brother who rarely prays, and I live with my husband's family. His relatives sit with him even if the imam is praying. What must I do, since I am not one of his mahram (relatives one may not marry)? And is there a sin on me for that, since I am unable to advise him?

A: If he does not pray, he deserves to be boycotted. Do not greet him or return his greeting until he repents, because abandoning the prayer is major shirk (associating partners with Allah), even if he does not deny its obligation, according to the most correct of the scholars' opinions. This is based on the saying of the Prophet (peace and blessings be upon him): It was narrated by Imam Ahmad and the compilers of the Sunan with a valid chain of narration. And his saying (peace and blessings be upon him): It was narrated by Imam Muslim in his Sahih.
As for if he denies its obligation, he is a disbeliever by the consensus of the scholars. It is obligatory for his relatives to advise him and to boycott him if he does not repent. It is obligatory to refer his case to the one in authority so that he is asked to repent; if he repents, well and good, otherwise he is to be killed. This is based on the saying of Allah (Glorified and Exalted is He): [Surah al-Tawbah: 5] and the saying of the Prophet (peace and blessings be upon him): "I was forbidden to kill those who pray." This indicates that one who does not pray is not to be left alone, and there is no objection to killing him if his case is referred to the one in authority and he does not repent. And success is from Allah.