ANSWER:

No, you won’t be sinful. 

However, while your father is still alive, tell him about the prohibition of what he has done and how he will be answerable in front of Allah for being unjust with his daughters and not distributing the gift equally among all his children. He’s your father, you don’t want this punishment for him in the hereafter.

My father had 100 acres of land , he is still alive, we 2 brothers and 2sisters, my father has already named the land half as my property and half to my brother, now officially in the records my brother and I am the owner, after my fathers death would I and my brother be held accountable for not giving anything to my sisters of that land ?although we never asked our father to do so?