If the console shows both "e" and "this" to be what you expect them to be, but "childClass.childNodes" is undefined, then something else is not what you are expecting it to be. Try
var rootParent = e.currentTarget.parentNode.parentNode.parentNode;
var childClass = this.siblings(rootParent);
if "rootParent" looks OK (eg. there are that many parent nodes and it returns it) then that would narrow it down to "siblings" not returning what you want it to.
If "rootParent" looks OK please post the code for the siblings function.