What are my rights

No.

Even if your mother did not have power of attorney, as the legal heir, she can sell the property, after the death of her father.

You Can  file  suite to claim the property for ur share. 

Dear Mr. Ramesh.

My Mother sold this property while my grand mother was alive without having her concent or power of attorney. Property si still on my grand fathers name

The property belonged to your grand father.

After his death, your grand mother and your mother are legal heirs. 

Your complaint is that your mother sold the property, without the consent of another legal heir,.ie your grand mother.

Yes, the sale may be voidable in respect of 1/2 share belong to your grand mother. But that ought to have been agitated by your grand mother.

In any case, neither you nor your brother is legal heir to your grand father's property, in the presence of your mother. Therefore you both can claim no right or share in the property. Consequently you cannot file any suit.

the said sale can be challenged only by grand mother. you can't challenge the same.

Dear Prasan,

My grand mother is no more alive

You say that the said deed is not registered by revenue authoritied and what is there for you to challange.You are successor of grandmother. However, your query is vague it does not spell the dates.  It is not cler as to when which event took place. Eviction of ocupant from land is not very easily evne if unauthorised.  The law is posseor freindly. You can sue the buyer as well as your mother. Even if you do not get the property you can atleast get a bad name for suing mother at old age which yu may consider as a prize possession, if you value the property more than your mother. You can even file criminal case on your mother and send her to Jail.