Property in mumbai

Gift/relinquishment deed means the property gets transferred immediately to her daughter, even though aunt is alive. Will takes effect only after her death. So she should decide what to do. 

She can write and register a Will wherever she is residing. Get it registered to avoid any dispute, registered Will always has better legal value than unregistered Will, though registration of Will is not compulsory.

If she dies without writing a Will or any other document, if the property remains in her name without will on her death, then all her children have a right to claim the property.

A Will can be made in Delhi for a property in Mumbai. But if the Will is made in Delhi the witnesses also likely to be residents of Delhi. I do not know exactly where the Will has to be probated. If it has to be probated in Mumbai then the Delhi witnesses may have to travel to Mumbai. It is advisable that your aunt clearly explains in the Will how the property was self-acquired to avoid disputes. There is heavy stamp duty on probates in Maharashtra. Besides writing a Will, it is very much advisable that your aunt nominates her daughter to receive the property from the Society after her death. The Society will have to transfer the property to the daughter's name on mere application without any hassles. Later the Will can be executed conveniently. If no nomination is made the transfer will have to wait until the Will is probated. If the other children dispute the Will, the process can get delayed.