input: Write a python function to get the siblings of a FamilyTreeNode.
  assert FamilyTreeNode('root').get_siblings() == []
  node_a = FamilyTreeNode('A')
  node_b = FamilyTreeNode('B')
  root = FamilyTreeNode('root')
  root.add_child(node_a)
  root.add_child(node_b)
  assert node_a.get_siblings() == ['B']
  assert node_b.get_siblings() == ['A']
output: class FamilyTreeNode:
      def __init__(self, name):
          self.name = name
          self.children = []
          self.parent = None
  
      def add_child(self, child):
          self.children.append(child)
          child.parent = self
  
      def get_siblings(self):
          if not self.parent:
              return []
          return [sibling.name for sibling in self.parent.children if sibling != self]