You didn't provide much to go on but, generally speaking, you can get sibling nodes
of the current node using something like this:
var children = get_parent().get_children()
Note, that collection will also include the current node (as it's also a child of its parent).
Regarding doing it every frame - you could just place the code in _update()
, though is that really what you want to do?
Perhaps a more detailed explanation of your goal would be helpful.