Partition of our inherited family property in Kerala

We are four siblings and own jointly an inherited family property of an old house, pond and land area of 1acre ( wherein the old house and pond are situated). We agreed upon for partition and divided the property in 4 portions namely A,B C with old house, D with pond . D being the last portion to get land area 15% more than A,B,C. For Pond included in D, we provided additional land space of 2/3rd of Pond area to D. An indicative drawing was made and all agreed to the same also . For access purpose, we provided two vertical roads between A &B and between C & D. We made a family agreement on a plain paper and each portions are assigned to 4 stakeholders through a fair lot process. After the lot process, assigned portion was mentioned against the stakeholders name with their signature and signatures on all pages by the stakeholders and two witnesses. But unfortunately, one of the siblings backtracked and filed a suit against other 3 stakeholders including me. Original plain paper agreement signed by all was misplaced or destroyed by the person who backtracked and we only have a photo of that agreement with us. Kindly be informed that we agreed to print the signed agreement on stamp paper subsequently which is mentioned in the plain paper agreement as an interim document till the partition documents are presented to the concerned Registrar and Registration of partition is made. We couldn't do that for the obvious reason of back tracking by one of the siblings. Now that he has filed a suit under section 26 Order 7 Rule 1, how we should go about defending our case. Please advise