Can gift be voided in this case

Hi

My mother gave me her house as a gift (gift deed) but that house was partly paid for by my brother and mostly paid for by my father. My brother who owed my father money. Now he is questioning the gift as he says he paid for it and therefore the the house could not be gifted. My father gave him other properties which were solely paid for by him. I cannot prove that he owed my father money as it was not a written debt.

Please advise if he has a case and what should I do.