Dear Mahesh,
First and foremost it is to be seen whether the property was self-acquired by your father or inherited by him from his father (i.e. your grand father).
If the property in question is a self-acquired property of your father, which according to you was given to your mother under a will, one has to know the terms and conditions of the will - whether the property will has been absolutely bequeathed to your mother or she can hold it till her life time etc. etc.
If the property had been absolutely bequeathed to your mother, then she can dispose of the same in any manner that she likes. She can gift it to any one she likes - no one else can object to it. However, the other interested persons like your brother and sister will always say that the gift deed has been obtained by exerting undue pressure, influence, coercion and by means of fraud etc. This you may have to defend.
If the property is not the self-acquired property of your father, then the will in your mother's favour itself is invalid. The property in that situation would belong to all of you i.e. your mother, you, your brothers and sisters in equal share.