$(".li2").prev().css("background","red");
$(".li2").next().css("background","red")
console.log($("#ul").find("li"));
<ul class="nva1">
<li class="li1">1</li>
<li class="li2">2</li>
<li class="li3">2</li>
<li class="li4">4</li>
<li class="li5">5</li>
</ul >
*{padding:0;margin:0;
list-style:none;
}