Originally Posted by
PuzzledHeart 
I think that a non-functional person will see that boundary and the protection surrounding it and will willingly slam against it. They'll beg you to tear down your defenses even as they continue injuring themselves.
This is very common scenario on this board. The A, usually the partner, will indeed slam themselves against the boundary and try to break through. As time passes, if they make no progress they usually stop trying and accept the new reality. This goes for everyone from toddlers to teens to adults.
If you're willing to set some boundaries, like I will never cover for my sister, I will not argue with her, I won't become enmeshed in her dramas, I will stop trying to solve her problems etc. the trick is how to stop her pressing your buttons. With an ex-partner it's mainly a decision to go non-contact but I assume this isn't an option for you.
If you decide on the boundaries, why not discuss with your therapist what phrases to use? I recently had to have a difficult talk with a hostile boss, and I actually asked the HR person how to phrase what I was going to say. It made a huge difference because I had them at hand when I was feeling upset and emotional.
My suggestion is to make a list of boundaries, talk them over with your therapist, and practice a few ways of dealing with your sister's pushing back. I promise she try and break you, but will give up eventually. Once she gets the message, you might find your relationship becomes one of mutual benefit.