Is a wife considered sinful if she prevents her husband from going to his second wife, especially if he is not equitable between the two wives?

If the other wife is still married to the husband, then he is obligated to give both wives their rightful maintenance according to what is customary, and to be just between them in dividing his time, unless one of them relinquishes some of her right. Preventing the husband from dividing his time for his wife, or inciting him to abandon her, is a sin; because assisting in injustice is forbidden.