Welcome stillnewtothis: I'm so sorry about the situation with your sister, and that she won't be your maid of honor. It has to be heartbreaking, as well as frustrating, to see her fail to live a full life.
Your sister may have BPD, or a combination of various issues (an anxiety disorder, ADD/ADHD, etc.). A professional would need to make a diagnosis. Is there any history of mental illness in the family?
I do not interact with her on a daily basis. However, I get the sense that she is trying to pull me into the same cycle my parents are in with the recent increasing number of emails. In general, I do not engage because I do not want to say or do anything to encourage unhealthy behavior.
Your approach to NOT engage is good and it sounds like you have an understanding about boundaries. If you go to the top of the page, to the wide green menu bar, you will see a "Tools" section. The links at that location, can lead you to a starting point to learn some more skills.
Every time we approach her about finding work so she can live independently on her own, she becomes very defensive and shuts us out. She also blames my parents for her inability to focus and find work.
Sounds like you parents are providing everything she would ever want. Wow, health insurance, an allowance, credit cards, food, shelter. . . Why would she ever want to leave? Sounds like she doesn't have any reason to get a job?  :)id she graduate from college with a particular degree, that might lead to a job?
Is there any chance you can talk your parents into getting a therapist for some family counseling. You and your parents can go discuss a possible plan to set boundaries with your sister? Perhaps with the help of a professional, your parent's might take some action and apply boundaries. You are right, nothing is going to change, unless your parents set some boundaries. The longer they wait, the worse it will be. If they wait too long, she may never be employable or self-sufficient in any regard.
You can't control you sister, or your parents. The only thing you can do is manage the way you interact with your sister and how you react to her. Boundaries will continue to be your friend, so stay consistent and keep adjusting them according to your needs and changing situations.