My brother-in-law only performs Salah (Prayer) during Ramadan.
I love him because he is a kind man, but I do not love him because he does not perform Salah. I have no authority at home to criticize him, because he is a stubborn man, and my father respects him a lot. I know the ruling on this marriage; that it is Batil (null and void), but my family is not aware of this. They mock me and other Multazims (practicing Muslims), and I have no authority to object to this. What shall I do? Please advise me and may Allah reward you with the best!
If you mean that this man does not offer Salah at all, it is not permissible to marry him to a Muslim woman, based on the saying of Allah (Exalted be He):
And give not (your daughters) in marriage to Al-Mushrikûn till they believe (in Allâh Alone)
And His saying:
...then if you ascertain that they are true believers, send them not back to the disbelievers. They are not lawful (wives) for the disbelievers nor are the disbelievers lawful (husbands) for them.
(Part No. 18; Page No. 294) As for you, you should advise the man, hoping that he might make Tawbah (repentance to Allah). If he does so, the marriage contract should be renewed. Your sister also should not allow him to be intimate with her until he makes Tawbah and renews the contract. May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.