Praise be to Allaah.
If the wife who is with you now is religious and of good character, then you are not obliged to divorce her. If you are able to treat both wives equally, then respond to your parents request to marry the cousin you mention. This is included in the words of the Quraan (interpretation of the meaning): then marry (other) women of your choice, two or three or four [al-Nisa 4:3]. If you fear that you will not be able to treat them equally and fairly, then be content with one wife, whether it is the first or the second, because Allaah says (interpretation of the meaning): if you fear that you will not be able to deal justly (with them), then [marry] only one [al-Nisa 4:3]. Try in all cases to please your parents. And Allaah is the Source of strength.