It's one of the alternatives ;)
Edit That alternative included the indefinite article, while yours didn't, so your suggestion has been added as well. Thanks for your comment!
What's the story with αδελφή vs. αδερφή, if you don't mind my asking? Is one considered a more antiquated spelling? Is one a regional variant?
Good question risingape. Αδερφή is an equally acceptable alternative form of αδελφή, which seems to be the formal one. In modern Greek they are equally correct. Can the Duo Greek team sort this???
Both are accepted. Except on the Listening exercises where of course you need to type what you hear. Usually, it's "αδελφός/αδελφή".
The word αδρλφός (brother) comes from the ancient greek word δελφύς (woomb).
"My sister herself eats a chocolate." is also a correct translation.
My sister herself eats a chocolate was marked wrong. The Greek is ....τρώει μία σοκολάτα
Nobody who is a native English speaker would say My sister herself eats chocolate. They would say, My sister eats chocolate. or My sister eats chocolate by herself..
Actually, both are correct but the meaning is different.
When we use the expression "My sister herself..." we are emphasizing the fact that she eats chocolate. E.g. "My sister never lets the children eat chocolate and tells them it's unhealthy....but my sister herself eats chocolate." As if this a contrary to the expected situation. "The Prime Minister himself answered the phone." not a secretary or an aide.
Saying "My sister eats chocolate by herself." means she does it when she is alone. No one is with her. Two completely different meanings.
Saying simply "My sister eats chocolate." is defeating the purpose since the grammatical phenomenon we are teaching here is the use of the emphatic "Η ίδια". There is a good explanation in the Tips & notes for this skill.
Btw I would say "My sister herself eats chocolate." or "I myself made that cake." "You yourself suggested the trip to Miami have you forgotten." And I'm a native English speaker and longtime English teacher.
See here for more on Reflexive Pronouns https://dictionary.cambridge.org/grammar/british-grammar/pronouns/pronouns-reflexive-myself-themselves-etc