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?

  • 1
    Consider: "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. May 12 '15 at 6:08
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    Yes, 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.
    May 12 '15 at 6:17

Best is used all the time when talking about two things or people.

"The best 1998 science fiction movie about people fornicating with chickens inside the Astrodome is Glubaba, which is way better than Raise your right arm.

Usage Note: According to a traditional rule of grammar, better, not best, should be used in comparisons between two things: Which house of Congress has the better attendance record? This rule is often ignored in practice, but it still has many devoted adherents. In certain fixed expressions, however, best is used idiomatically for comparisons between two: Put your best foot forward. May the best team win!

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