Your letter badly needs editing!
Good on you for caring enough about your sister to try to reach her with a well-reasoned letter. However, it is way too long! Can I share with you some advice I received from a course co-ordinator when I submitted one of my first university assignments. She wrote: "Steve, you write really well. But how are readers ever going to see how well you write when your assignments are so long-winded? Potential readers will probsbly not even bother to carefully read it when they see long pages full of typed information".
As much as I inwardly reacted to this negative feedback, I got the point and have never forgotten it.
So, I ask you, Why bury your excellent points in a mass of words? Imagine your sister first seeing this extremely long letter. Just its length alone would overwhelm her. Prove that you definitely want her to read it by limiting your letter to only one or two key points and back them up with some concisely presented evidence. Then close. It will be powerful - and even against her better judgement, she'll be more inclined to read it in its entirety. 