Final answer:
The sister likely wrote the long and confusing letter based on the narrator's frustration and desire for her to be a better communicator.
Step-by-step explanation:
Based on the provided paragraph, the sister most likely wrote the letter that is described as an encyclopedia and excessively long, with a list of requests that is difficult to untangle. The narrator expresses frustration towards their sister due to the challenges faced in understanding and acting upon the details in the letter. The final sentence, "If only my sister were a better communicator!", implies the sister's role in authoring the letter and her lack of clear communication skills.