Our mother, who lives in another city with our sister, without informing us of her intention and in connivance with one of our female cousins and her husband got our sister married to a non-Muslim boy of her choice. We do not know whether the boy has embraced Islam and whether the wedding was solemnized in Islamic way. While we have broken all relations with the cousin and our sister, we need guidance in the view of Islam on how to treat our mother. We, the brothers, are very angry with her because of this act.

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.

If your sister is married to a non-Muslim man, then her marriage is invalid and void, without any difference of opinion among the scholars about this, and they must be separated. Please refer to fatwas 262615, and 81220.

You must denounce what your sister and those who helped her in this evil did, and it is permissible for you to cut the ties with them – with the exception of the mother – as a way of deterring them from this evil.

However, the benefit should be taken into account; if cutting the ties with them will bring them back to the truth, then it is better to cut them with them, but if what is more beneficial is to keep the ties with them and continue to advise them, then it is better to keep ties with them. Please, refer to fatwa 347560.

As for the mother, it is not permissible to cut the ties with her, and you must to be kind and dutiful to her in all cases.

Shaykh Ibn Baaz said, "What is meant is that the parents are of great status. The child must not desert them. Rather, he should give them advice and guide them to what is good in a soft and gentle manner, and in doing so, he may seek the help of maternal uncles, brothers, paternal uncles, etc." [Excerpted from Fataawa Noorun ‘ala ad-Darb of Ibn Baz]

For more benefit, please refer to fatwa 85041.

Allah knows best.