بسم الله الرحمن الرحيم
In the above mentioned case the will of your father shall not be valid and all the heirs shall get their shares in the flat of Mumbai and other property. It shall not be valid to divide the flat of Mumbai only between one son and daughter. It should be noted that this ruling shall be applicable only when your father has died; otherwise there is no need to ask question about inheritance of the property as your father is the sole owner of all his property throughout his lifetime.
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.