Is it obligatory to build another mosque for a wealthy deceased person who made no bequest, after his family built a mosque for him from his inheritance and one of them saw in a dream someone asking for a mosque to be built for him?

The deceased's estate is the property of the heirs. If they are adult and of sound judgment, and they voluntarily choose to build a mosque, this is commendable. However, if there is a child or a person of unsound mind among them, it is not permissible to dispose of their share of the wealth. Rather, their rightful share of the inheritance must be placed under the care of a trustworthy guardian who will preserve and grow it for them until they reach adulthood and maturity, at which point they can dispose of it as they wish.