[0] First not to assign id a numeric. It is no good, period. Also [tt]<li="2">[/tt] is not serious.
[1] Suppose you have this.
[tt]
<ul id="id1">
<li id="id2">bla</li>
</ul>
<ul id="id3">
<li>bla</li>
</ul>
[/tt]
[1.1] Starting from assumption you've got the reference to id="id1", via say getElementById() method. Then you can do this.
[tt]
var onode, otarget;
onode=document.getElementById("id1");
while (onode) {
onode=onode.nextSibling;
if (onode.nodeType==1) {
otarget=onode;
break;
}
onode.nextSibling;
}
if (otarget) {
//you actually have found one, and do something here
alert(otarget.id + "\n" + otarget.tagName); //just to verify
} else {
//you don't find one
}
[/tt]