What do I do if my husband marries another woman?
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.
The noble questioning sister should know that Allah has made lawful for a husband to marry more than one wife if he is able to fulfill the marital obligation and can be just when dealing with his wives. This is the Command of Allah in His Book and there is none to put back His Judgment. Allah Says (what means): {Does He who created not know, while He is the Subtle, the Acquainted?.} [Quran 67:14]
The clear evidence for this permissibility is the verse of the Quran in which Allah Says (what means): {…then marry (other) women of your choice, two or three, or four but if you fear that you shall not be able to deal justly (with them), then only one or (the captives and the slaves) that your right hands possess.} [Quran 4:3]
Everyone who believes in Allah should submit himself/herself to the religion of Allah without any objection or hesitation. Allah Says (what means): {It is not for a believing man or a believing woman, when Allah and His Messenger have decided a matter, that they should [thereafter] have any choice about their affair. And whoever disobeys Allah and His Messenger has certainly strayed into clear error.} [Quran 33:36]
Allah knows best.
You can search for fatwa through many choices