Quick:
var siblings = n => [...n.parentElement.children].filter(c=>c!=n)
https://codepen.io/anon/pen/LLoyrP?editors=1011
Get the parent's children as an array, filter out this element.
Edit:
And to filter out text nodes (Thanks pmrotule):
var siblings = n => [...n.parentElement.children].filter(c=>c.nodeType == 1 && c!=n)