Seller sold jointly owned land for which mutation is not yet done

Respected sir, I have buyed a 10 dhur plot few days back but I came to know that the plot holder is actually a mental and also bifurcation of the total plot of 30 dhur in between them(plot holder and his younger brother ) has not yet done.But while registry of the plot he accepted that he sold the plot and also accepted that he also have received the sum amount in total .Sir the plot holder have one son , a girl (married) and wife so , both of them his son and his wife witnessed while registry .I only want to know that in future will it affects me or not if it creates any problem for me then kindly show me the right way .