"It will be my pleasure."
Translation:Θα είναι ευχαρίστησή μου.
Yes, unless you want to say that she is your smaller sister or the sister you were talking about etc.
-Είναι η κοπέλα σου;=Is she your girlfriend?
-Όχι, είναι αδερφή μου=No, she is a sister of mine. (you could even say that if you have only one sister, but at this point of the conversation you only want to talk about your relation)
Shouldn't there be a second τόνος here? Such as: Θα είναι η ευχαρίστησή μου.
Yes, there should.
Unfortunately, the current version of the course dates back to a time where it was impossible to use a word that "hadn't been taught" -- and since Duolingo considered ευχαρίστηση and ευχαρίστησή to be "separate words", and only the first of those was in the course, many sentences use words which are incorrectly accented only once, not twice.
Also, the "best" answer for an English-to-Greek translation exercise always has to be (or at least has to include) the version that is used for Greek-to-English translation -- which often means, the version without the correct second accent.
The version with a second accent is accepted (and if not, please report it -- the intention is to accept all of the correct spellings, though some may have been missed) but it's not presented in "pick the correct translation" or on the sentence discussion page because it's not the "best" translation.
This is one of the points that will be improved on in the next version of the course. (Which has no ETA yet.)