We say we use 'the' only with superlatives. So is this correct;
Preeti is prettier than her sister.
Preeti is the prettier of the two sisters.
The superlative degree denotes the most, the largest, etc., by which it differs from other things. Therefore the superlative degree used in the mentioned second sentence is wrong. Two things are always compared and thus, comparative degree is used while superlative degree is used for more than two things. Thus, the correct form of the sentence could be formed only if there would have been three or more than three sisters.