Equal payment made to buy a house, what are my rights.

My uncle and my father bought a flat in 2006 ,my father paid 2.5 lakhs and my uncle paid 2.5 lakhs and 1 lakh for registry, thereafter agreement was made, after paying EMI for 8 years @ 5100 per month my father paid last lum sum of rs .3 lack to back against loan principal and now the running pending loan is of around 1.5 lack. now my wants to built a separate kitchen bathroom in the flat .I am not in the support of the same but they are claiming that they paid equal share of the cost of the house. he showed that he paid for furniture and all which might cost 2lakh. what shall I do legally.