You can use:
if (parentGuest.nextSibling) {
parentGuest.parentNode.insertBefore(childGuest, parentGuest.nextSibling);
}
else {
parentGuest.parentNode.appendChild(childGuest);
}
But as Pavel pointed out, the referenceElement can be null/undefined, and if so, insertBefore behaves just like appendChild. So the following is equivalent to the above:
parentGuest.parentNode.insertBefore(childGuest, parentGuest.nextSibling);