Translation:I am her younger brother.
Almost, but not quite. 「わたしは」 is "I" (the verb is at the end), 「かのじょ」is "she", it is the following「の」that turns it into "her", and「です」at the end is (or functions as) "am". A word-for-word translation would give "I her younger brother am", so more properly "I am her younger brother."
わたし (私): I, me; は: particle which indicates the topic of the sentence; かのじょ (彼女): she, her; の: particle which indicates a possession (for example, 私の犬; 犬 is a possession of 私); おとうと (弟): younger brother (it is always used like this); です: well... I hope you already know this one...
I entered exactly what the correct solution is and it marked me as wrong. I triple checked by taking a screenshot and moving the sentences around to line up my given answer and the correct solution. There is no bug report option for "I should be correct" at the time of writing this.