How to find second last element from a List?

Starting with C# 8.0 you can use Index to get access to elements relative to the end of sequence:

if (lsRelation.Count >= 2)
    secLast = lsRelation[^2];

See docs for more information

If you know that's an IList<T> which has an indexer:

string secondLast = null;
if (lsRelation.Count >= 2)
    secondLast = lsRelation[lsRelation.Count - 2];

You could create an extension like:

public static T SecondLast<T>(this IEnumerable<T> items)
{
    if (items == null) throw new ArgumentNullException("items");
    IList<T> list = items as IList<T>;
    if (list != null)
    {
        int count = list.Count;
        if (count > 1)
        {
            return list[count - 2];
        }
        else
            throw new ArgumentException("Sequence must contain at least two elements.", "items");
    }
    else
    {
        try
        {
            return items.Reverse().Skip(1).First();
        } catch (InvalidOperationException)
        {
            throw new ArgumentException("Sequence must contain at least two elements.", "items");
        }
    }
}

Then you can use it in this way:

string secondLast = lsRelation.SecondLast();

Use:

if (lsRelation.Count >= 2)
    secLast = lsRelation[lsRelation.Count - 2];