What is the obligation toward a brother who does not pray?

Question:
Your Eminence: Peace be upon you and Allah's mercy and blessings. After that: I love you for the sake of Allah the Exalted very much, and I hope that Allah the Exalted will make us and you among those who love one another for His sake. Amen.
After that: I have a brother from the same mother and father who does not pray in the mosque, and he does not live with me. He wants to get married, and my mother asked me to go with him to propose a marriage for him, but I refused, saying: No, because he does not pray. However, when he prays and the imam of the mosque testifies to it in his home, then I will go with him and propose for him. But, O Shaykh, now my mother does not speak to me, and all my brothers and sisters do not speak to me and have cut off ties with me. And praise be to Allah in any case.

Answer:
As for love for the sake of Allah, we say: May Allah love you, the One for whose sake you loved us, as it has been mentioned that love for the sake of Allah is among the best of the nearness to Allah.
As for what you mentioned about your brother: It is obligatory for you and all your brothers to advise him, and for you all to maintain contact with him, not just you alone; rather, others should be with you so that it may have an effect on him, and so that he may respond to the call to truth. Because if you are alone, he may not care, especially if he is older than you. The point is that you should follow up with the advice, and others from your good brothers or your neighbors should be with you; perhaps he will accept from you and pray and maintain the prayer in congregation. Then you can fulfill the wish of your mother and your brothers in helping him and going with him to the place from which he is proposing, after Allah guides him to this matter and he accepts it from you and prays.
As for if he insists on not praying, do not go with him, but boycott him until he repents to Allah and returns to the right and the truth, even if your mother is angry and even if your brothers are angry. Because the one who does not pray is a disbeliever in the most correct of the scholars' opinions. Whoever abandons the prayer and intentionally abandons it has committed disbelief, as the Prophet (peace and blessings be upon him) said in the authentic hadith: And he (peace and blessings be upon him) said: in many other hadiths that indicate this.
As for his abandoning the prayer in congregation, this is easier; he may have a doubt, because some scholars do not see the congregation as obligatory, so there is no harm. You should maintain contact with him along with your brothers and strive for his prayer in congregation if he prays but does not pray in congregation. This is easier; so you should maintain contact with him along with your brothers and others from your neighbors and those who help you in this; perhaps Allah will guide him through these means; so that you combine all the benefits, between the pleasure of your mother and your brothers and Allah's guidance for him through your hands and the hands of your brothers who accompany you in calling him to Allah . May Allah grant everyone success and guidance.