You can get any node in the tree by providing the correct path. Node paths work just like folders on your operating system.
get_node("..") gets the node one level up and is equivalent to
Let's say you have the following setup:
| +-- ChildA
Your script on "ParentB" wants to get "ChildA". The path is
.. (goes up one level to "NodeA") followed by "ParentA/ChildA" You could write this in two ways:
var node = get_node("../ParentA/ChildA")
var node = get_parent().get_node("ParentA/ChildA")