Is a married man obligated to obey his mother in not marrying a second wife, and if he marries without her consent, is he committing a sin?

It is permissible for a man to marry up to four women, provided he is able to treat them justly, based on the Almighty's saying: ﴿Marry such women as seem good to you, two, three, or four; but if you fear that you will not be able to deal justly (with them), then only one, or (from) what your right hands possess. That is more suitable to prevent you from injustice.﴾ The consent of parents is not a condition for the validity of the marriage, but effort should be made to convince them. If the parents insist on prohibiting it, it is better to forgo the marriage in seeking their pleasure, for "the Lord's pleasure is in the parents' pleasure, and His displeasure is in their displeasure."