Hi frostysister
Congratulations on your upcoming wedding

You had quite a difficult childhood as a result of growing up with a BPD sister.
I know that this hurts my parents because they want me to help support her and for us to be closer. This is a main source of guilt for me. They have a very difficult time and I feel like I only make it harder on them.
Though it might be hard on your parent, I do want to say that it is not your responsibility to support your sister. You can choose to do so if you want to, but you most definitely aren't obliged to if it is at the expense of your own well-being. Your feelings matter too and your childhood and current situation with your family of origin isn't easy.
There are tools that might help you better interact with your sister. Since you have already been aware of BPD for quite some time, you might already know them but I am going to list them anyway:
Validate, don't invalidate, but only validate the validExpress your truth - S.E.T.: Support, Empathy, TruthAssert yourself - D.E.A.R.M.A.N.: Describe, Express, Assert, Reinforce, Stay Mindful, Appear Confident and NegotiateThese tools don't offer a 100% guarantee of success, but following the scripts can increase your chances of success and can also help you stay more calm yourself. These structured ways of communicating help reduce the likelihood of (further) conflict while maximizing the likelihood of getting through to the other person.
You were a child when your sister was diagnosed with BPD, did she ever get any targeted treatment for her BPD back then? Is she currently receiving any treatment for her issues?
Take care and I hope to hear more from you later
The Board Parrot