Land purchased, adjoining lot occupied a portion of this land

I purchased a land 300 sqm residential with 185 sqm orchard, total of 485 sqm. As per declaration of deeds, it shows that the total land area is 485 sqm, a portion of which has been taken by adjoining lot with house structure. The house was built long years back on the adjoining lot, part of this land where this house stands is a portion of the 185 sqm orchard. The seller and the adjoining lot owner are relatives. The land purchased is a part of the whole lot owned by their grandparent which was not previously surveyed nor titled before sale as they only possess each a declaration paper. What is my remedy and action?

Had it surveyed and titled. Hire a Geodetic Engineer.