treeview text highlighted after a video clicked

[tayss]

2. Header << Play Video (a) [Duration 0:00 to 0:030] and (b) [Duration 0:30 to 0:41] on click

a) sub header 1 << Play Video (a) only on click
b) sub header 2 << Play Video (b) only on click

When I click 2. Header it was default highlighted blue background with white foreground.
So 2. Header is playing videos on (a) and (b) after I click on 2. Header. There will be a slider to detect the duration.
So now, I want a) sub header 1 to be highlighted when the slider is at part A video [2. Header is still showing video (a)]
Same for b) sub header 2 to be highlighted when slider is at part B video.[ 2. Header is still showing video (b)]



And I did this code...

private void SeekToMediaPosition(object sender, RoutedPropertyChangedEventArgs<double> args)
  {
   _timer.Stop();
   TimeSpan t = TimeSpan.FromSeconds(VideoTime.Value);
   Video.Position = t;
   _timer.Start();

   if (t.TotalSeconds < 30)
                {
                    twoA.IsSelected = true;
                }
                else if( t.TotalSeconds <41)
                {
                    twoB.IsSelected = true;
                }

      else
    {
     threeB.IsSelected = true;
    }
   }
  }
And I didn't get what I wanted because, at only the

if (two.IsSelected == true)
            {
                if (t.TotalSeconds < 30)
                {
                    twoA.IsSelected = true;
                }
                else
                {
                    twoB.IsSelected = true;
                }
            }

i already dun see the isSelected function to go to twoB after 30sec later... Any clue? ):
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