JQuery :: How To Get Current Sibling Number

I've made a function for getting a ID for each sibling in a tree. Lets look at this DOM:
<ul>
<li>Five</li> //sibling ID 1
<li>Six</li> //sibling ID 2
<li>Seven</li>//sibling ID 3
</ul>

Here is the function:
function getSiblingId(elm) {
return $(elm).siblings().size() -$(elm).nextAll().size() + 1;
}

And here is how I use it:
var id = getSiblingId($('ul li:first-child')); // id = 1
Is there a more elegant way to get the same result? The work done in the getSiblingId-function has a lot of overehead.