Definite Article in front of the word 'better' [duplicate]

Is it wrong to have 'the' in front of the word 'better'? Can someone help me sort this confusion out? Case in point: "Ella is the better of the two sisters" vs "Ella is the best of the two sisters". Grammarbook.com says that the former is correct and I agree. But a friend brought up an interesting point stating that one should never use 'the' in front of the comparative degree. But how about sentences like :

• Who is the better of the two?
• This is the better option.
• Be the better person.

As far as my knowledge goes I do not see anything grammatically incorrect in these sentences/questions and I believe the choice of words in this scenario depends on the context. Am I correct?

the + comparative adjective when only two things are involved:

He's the taller/better/wiser of the two.

"He is the taller of the two brothers."

http://www.enotes.com/homework-help/2-he-taller-two-brothers-3-he-would-make-better-341486