Q: When a husband marries an additional wife is it compulsory for him to inform the wives of each other?
A: If he does not inform them it does not affect the validity of either marriage, but it is his moral obligation to inform them. If he does not, it can lead to complications later, e.g. if he dies then there can be a dispute with regard to the inheritance, etc.
Moulana Yusuf Laher