[jQuery] select the last 2 elements in list

[figo2476]

e.g.

<ul>
<li>a</li>
<li>b</li> <-
<li>c</li> <-
</ul>

I did:
<script type="text/javascript">
                $(document).ready(function() {
                   $("#update li:last").addClass("last_li");
                   $(jQuery.sibling("#update
li:last").prev).addClass("sec_last_li");
                });
</script>

The last element works, but not the 2nd last....