I have a brother-in-law who rarely prays. I live with my husband’s family and [ the female members of] his family sit with him even if the Imam is praying. What should I do? I am not one of his relatives (mahram). Is there any sin upon me since I do not have the ability to advise him?
If he does not pray, then he is deserving to be boycotted. You should not greet him nor should you respond to his greeting, until he repents. This is because not praying is a greater form of kufr [that takes one out of the fold of Islam]. This is true even if a person does not deny that it is obligatory. This is according to the most correct opinion among the scholars. The Prophet (sallallaahu ‘alaihi-wasallam) said,
“The covenant that is between us and them [the disbelievers] is the prayer. Whoever abandons it has committed blasphemy.” This was recorded by Ahmed and the compilers of the Sunan with a sound chain.
The Prophet (sallallaahu ‘alaihi-wasallam) said, “Between a man and disbelief and polytheism is the abandoning of the prayer.” This is recorded by Imam Muslim in his sahih.
However, if a person denies the obligation of the prayers, then he is a disbeliever according to the consensus of the scholars.
Therefore, it is obligatory upon his family to advise him and to boycott him if he does not repent. It is obligatory to take his matter to the ruler for him to be asked to repent. If he repents,[that is accepted from him]. If he does not repent, he is to be killed. This is because Allah has said in the Qur’an,
“But if they repent and offer prayer perfectly and give zakat, then leave their way free.” (at-Tauba: 5)
Also, the Prophet (sallallaahu ‘alaihi-wasallam) said,
“I have been prohibited from killing those who pray.” 
These evidences show that one who does not pray is not to have his “way left free” and there is no prohibition against killing him if he is taken to the authorities and he does not repent. And Allah is the One who provides guidance.
. Recorded by Abu Dawud. Some of the narrators in the chain are unknown. However, due to supporting evidence, al-Albani has called this hadith sahih.See Muhammed Nasir al-Din al-Albani,Sahih Sunan Abu Dawud (Riyadh: Maktaba al-Tarbiyah al-Arabi li-Daul al-Khaleej,1989), vol. 3 p. 931. He discusses it in some detail, although without mentioning that it is recorded by Abu Dawud, in Muhammed Nasir al-Din al-Albani,Silsilat al-Ahadith al-Sahiha (Riyadh: Maktaba al-Maarif,1991), Vol. 5, p. 493
Shaykh `Abdul-`Azeez Bin Baz
Islamic Fatawa Regarding Women – Darussalam Pg.34-36