JSFiddle - React, Tailwind, and code Playground

HTML

<ul>
  <li>11</li>
  <li>12</li>
  <li class="a" id="x">13</li>
  <li>14</li>
  <li class="a">15</li>
  <li>16</li>
</ul>

JavaScript

const startElem = document.getElementById('x');

// jQuery fill background with yellow
$(startElem).siblings().css('background', 'yellow');

// vanilla js add outline
myGetSiblings(startElem).forEach(el => 
  el.style.outline = 'dashed 1px gray'
);


function myGetSiblings(startElem) {
  return Array.from(startElem.parentNode.children)
    .filter(el => el !== startElem);
}