[Jennifer] and her sister [Mary] are the only children in their family. [Dennis] wanted to get a good gift for his sister, [Mary]
('Dennis', 'Jennifer')
[Dennis] wanted to get a good gift for his sister, [Mary] [Jennifer] and her sister [Mary] are the only children in their family.
[{('Dennis', 'sister', 'Jennifer'): [('Dennis', 'sister', 'Mary'), ('Mary', 'sister', 'Jennifer')]}]
['sister', 'sister']