What is the legality of land deal between father and son

My father bought a land 18 years back (in 1999) on his name and of his own income and income of his father in law(means he took the money as loan from his father in law without any interest).But he didn't paid the whole amount back to his father in law till jan. 2011. In feb 2011 my elder brother told my father that he want to buy a land anywhere in bihar , then my father told my elder brother to buy his land and pay him the money as he has to pay the money back to his father in law.On this my elder brother agreed.Then in the last week of feb 2011 my father sold that land to my elder brother and me jointly.then he paid the remaining amount back to his father in law.I want to know that this land deal between father and son is legal or not.What will happen if someone files a case against us that how a father can sale his land to his own son.we are hindu.