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 (saw) is His slave and Messenger.
It is forbidden for the Muslim woman to get married to a non-Muslim man because non-Muslims should not have any power over the Muslims.
So, you should do your best to nullify this invalid marriage. Try to contact the leaders and Imams of your community and to stop this grave sin. You should make the Sharee’ah ruling over this issue clear to your relatives.
Remind them of Allah, advise them, and tell them that this act is obnoxious and obscene. They should not keep silent as though nothing bad has been done. Tell them that if they participate in this act, they will commit a grave sin.
We hope Allah will guide you all to the right path.
For more benefit on the absolute prohibition for a Muslim woman marrying a non-Muslim man, please refer to Fataawa 90619, 87151, 274252, 124877, 84265 and 88406.
Allah knows best. (Source: islam_web) Classified in Women and Family > Marriage Contract