My father made me 100% owner of the flat under unregistered but duly witnessed Will. My brother and sister have signed Release Deed in my favour. Is it necessary to get probate of the will or get succession certificate? If probate is not taken or succession cert. not taken then will it be problem whilst selling the flat? 

Whether probate is required or not depends upon where the property is situated. Probate is required in Bombay and Madras.

Sir,

With due respects, any landed property/estate can not be owned without a legal instrument.  How are you made a 100% owner of the flat? Is it purchased in your name? or are you a nominee?

Nominee is only a trustee and does not own the property. The WIll has to be probated if you want to own the flat absolutely. The release deed of your brother and sister has to be executed and registered for it to have any legal meaning.

regards,

Kishor Mehta