How can a bonafide purchaser reply to a summon of title suit

My mother has purchased a land in 2016 after due enqiury about title and possession of the seller. Registration was done and also mutation was done in 2016. Possession was transferred to my mother in 2018. But, when she started construction on the plot of land, some relatives of the seller filed TITLE SUIT against 8 parties out of which, my mother is one of them. My mother received the summon. Please suggest how to reply to the summon and what grounds under which law to be taken.