using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        List<RelationDTO> relationDTOList = new List<RelationDTO>

            {

                new RelationDTO { PersonId = 1, RelativeId = 2, Relation = "Son" },

                new RelationDTO { PersonId = 2, RelativeId = 1, Relation = "Father" },

​

                new RelationDTO { PersonId = 1, RelativeId = 3, Relation = "Mother" },

                new RelationDTO { PersonId = 3, RelativeId = 1, Relation = "Son" },

​

                new RelationDTO { PersonId = 2, RelativeId = 3, Relation = "Husband" },

                new RelationDTO { PersonId = 3, RelativeId = 2, Relation = "Wife" },

            };

        var result = (from child in relationDTOList

                     join parent in relationDTOList on child.PersonId equals parent.RelativeId

                     select new Relations()

                      { 

                          PersonId = child.PersonId,

                          RelationId = child.RelativeId,

                          Relation = child.Relation,

                          ReverseRelation = parent.Relation

                      }).ToList();

            //var grp = relationDTOList.GroupBy(x => x.PersonId == x.RelativeId && x.RelativeId == x.PersonId)//(x => new { x.PersonId, x.RelativeId })//(x => x.PersonId == x.RelativeId && x.RelativeId == x.PersonId)

                                 //    .ToList();

            foreach(var g in result)

            {

                Console.WriteLine(g.PersonId);

                    Console.WriteLine(g.RelationId);

                    Console.WriteLine(g.Relation);

                    Console.WriteLine(g.ReverseRelation);

                Console.WriteLine("------------------");

                //Console.WriteLine(String.Join(",",g.Select(x=>x.Relation)));

            }

    }

}

public class RelationDTO

{

    public int PersonId { get; set; }

    public int RelativeId { get; set; }

    public string Relation { get; set; }

}

​

public class Relations

{

   public int PersonId { get; set; }

   public int RelationId { get; set; }

   public string Relation { get; set; }

   public string ReverseRelation { get; set; }

}