Get the first two distinct successors of a string from List

Get the first two distinct successors of a string from List

Consider the following example
List<string> subList1 = new List<string>();
subList1.Add("start");
List<string> subList2 = new List<string>();
subList2.Add("start");
subList2.Add("string1");
subList2.Add("string2");
List<string> subList3 = new List<string>();
subList3.Add("start");
subList3.Add("string3");
subList3.Add("string4");
subList3.Add("string5");
List<string> subList4 = new List<string>();
subList4.Add("start");
subList4.Add("string3");
subList4.Add("string6");
List<List<string>> mainList = new List<List<string>>();
mainList.Add(subList1);
mainList.Add(subList2);
mainList.Add(subList3);
mainList.Add(subList4);
So now if my
search string ="start" , output = {"string1" , "string3"}
search string ="string3" , output = {"string4" , "string6"}
Any suggestions?