[Jeff] bought a book for his father [Wayne]. [Wayne] bought a book for his son [Robert].
('Robert', 'Jeff')
[Wayne] bought a book for his son [Robert]. [Jeff] bought a book for his father [Wayne].
[{('Robert', 'brother', 'Jeff'): [('Robert', 'father', 'Wayne'), ('Wayne', 'son', 'Jeff')]}]
['father', 'son']