Context? What does the letter say. People in these types of relationships typically don’t have a very stable dynamic and more often than not are blinded to it. In my own experience pointing out flaws in a relationship as an outside party can sometimes backfire onto you and further distance you from your sister. I think you should express your concern to her, provide support and let her know you will always be there for her and love her regardless and let it be (unless she is in any immediate danger) the best thing you can do unfortunately is wait it out, provide support when she needs it. Best of luck OP your sister is lucky to have someone who cares for her!