Partition Of Property In Already Constructed Ground Floor

If parents died without making a will and the only son is living in the said house now two married sister also want their share and filing partition suit ..is the son also liable to file partition suit ( he is already in possession of the said house ) or is it understandable that he is already having his share in ground floor and sister can construct first and second floor eith their own money or can the court order to sale off the house ..in that case who will compensate the amount he already spent on construction of ground floor with his money when the parents are alive