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Of the two sisters she is the best.
Because there are only two sisters, 'better' should be used but can we, in some instance use 'best' in this sentence? Would it still be grammatically correct?
Is there a rule which specifies when we are allowed to use superlative degree while comparing two people/things?
1Consider: "This is the best book I've ever read." "How many books have you read?" "One." -- Just because it's possible doesn't mean that we should use it. – Damkerng T. – 2015-05-12T06:08:10.007
1Yes, your example is grammatical for a set of two comparison: though, it is generally considered to be (usually) restricted to informal style. -- CGEL page 1162, [2] and [3]. – F.E. – 2015-05-12T06:17:09.210