You could always do DListView:Clear() and then reinsert the lines, which is probably the easiest way.
Alternatively, you could do a loop for all of the lines that are after the index that you just removed and just decrease the index value by one.
For example, something like this (untested):
function PANEL:RemoveAndResort( x )
self:RemoveLine( x )
for i = x, #self:GetLines() do
line = self:GetLine( i )
line:SetValue( 1, line:GetValue( 1 ) - 1 ) -- assuming index 1 is the visible numeric index
If you remove the line at index 2, then the line at index 3 becomes the new index 2. So, using the same index of the line you just removed, this loops through all of the remaining lines from that index and decreases their visible index value by 1 manually.