Can property be willed to sister even if wife,son,daughter e

There should be a strong circumstance to dislodge his legal heirs. Further the execution  of will under influence of drug is purely a fact which should be proved by way of adducing unimpeachable evidence. But at the same time the propounded should remove all suspicions surrounding the xecvution of the will. As far as Evidence Act is concerned the will should be proved by examining one of the attestors regarding due execution 

How did the mother acquire the flat: it was self acquired or from her parents or from her husband?

Which personal law applies e.g. Hindu Succession Laws?

How did mother gove the flat to son; e.g. by WILL/ registered Gift deed etc?

Share full information.

Repeated Query:

https://www.lawyersclubindia.com/forum/Ancestral-property-121221.asp#.VVoPLFJ-hkg

The flat was self acquired.

This query was addressed properly by many experts/lawyers in the previous thread too. You may note that the wife and children can file a suit to declare such transfer like gift deed in favor of his sister or nephew or niece only after his demise and not before that.  They can challenge that transfer itself of having got it through coercion or fraudulent means but they have to prove that. Therefore secure evidence now itself for challenging the gift deed at a later stage.