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She sometimes makes her younger sister write her reports for her.
Literal
She [topic-は] sometimes younger-sister [causee-に] report [object-を] make-write.
The causative 書かせる ('make/let write') marks the causee 妹 with に — the standard, neutral case marker for the person caused to act on transitive verbs (since を is already taken by the object レポート). Switching に to を on a transitive causative would push the meaning toward coercion ('forced her sister to write'). レポート is a loanword from English 'report,' but in Japanese student life it specifically means a written paper or assignment for a class — so the sentence implies the older sister is offloading her own coursework. ときどき hedges it as occasional rather than habitual, which softens what is, after all, a confession.