[Mary] was spending the weekend with her grandson, [Robert]. [Robert]'s sister [Margaret] was supposed to come too, but she was busy and could n't make it.
('Mary', 'Margaret')
[Mary] was spending the weekend with her grandson, [Robert]. [Robert]'s sister [Margaret] was supposed to come too, but she was busy and could n't make it.
[{('Mary', 'granddaughter', 'Margaret'): [('Mary', 'grandson', 'Robert'), ('Robert', 'sister', 'Margaret')]}]