First off, she's coming to defend scientology. She's expecting a fight.
She's 'armed for bear', and ready to do anything but listen. You aren't going to win a fight. She's too indoctrinated, too stubborn, and too invested in scientology to up and say 'Ya know what? This is nuts!'
I don't know what's happened thus far, but if it's not too late, I would try a little jujitsu. Use her own energy against her, like popping a door open just as someone is throwing their full weight against it to batter it down.
She's expecting resistance and hostility. Give her love and acceptance.
When scientology was going bad for me (and so many others) the most terrifying part was that we had pretty much burned our bridges (no pun intended) back to the real world. There was no place to go to! Leaving a life that had a whole bunch of us all working towards a common worthy goal, with a common philosophy, a common way of doing things, and looking at the world was hard to contemplate.
Having nothing and no one to fall back on in the outside, well...
When she leave she needs to KNOW that you are her sister, you love her dearly, and you will be there for her no matter what.
She needs to believe that although you may not agree with scientology, or 'get it' yourself, that you respect her decision to go into and stay in scientology.
Acknowledge any good it has done in her life, and let her know that should it ever reach a point where it stops benefiting her, or she simply needs to get away from it to think about it for a bit (if the phrasing isn't too awkward for you say 'if you need to examine it from an exterior viewpoint') , that you love her and she is welcome there.
OK, that sets the table. I can only offer you a few things to put on the menu.
The writings of L. Ron Hubbard are like the Bible, in that one can find something in them to support darn near anything. You need to give her a little - just a little - cognitive dissonance. That is, gently hand her a few internal inconsistencies to mull over in her own time.
This is tricky.
From a church politics side:
The stated goals of the church are totally unreachable under the current system. The big one you'll hear is to 'Clear the Planet' at $1000+ per hour of auditing, this simply isn't going to happen, as that is a year's income for most of the world! Well, the usual answers are (were) that as more and more people are cleared, there will be less insanity (group bank) to go around and it will take less auditing time, and at that time the church will also be reducing the rates to draw in more and more of the world's poor. If you get this one, ask if the church has ever reduced rates? (no)
The other was that Hubbard was developing group processes to clear stadiums full of people at a time. I doubt you'll hear this one. Where is the old guy these days?
In either case, why the rush? Why not simply wait until it becomes more universally available and cheaper? Usual answer is that it's a race between scientology and the forces that would destroy the earth. Hard to argue that point, you're on your own here!
On the tech side two main things come to mind:
1) One of the very basic tenets of both dianetics and scientology is that you never ever ever tell the PC (Pre-Clear - client) what incidents to run in an auditing session. You have particular processes and commands, but the PC locates their own incidents that those commands point to. Your incidents of 'a pain in the zorch' (a made up Hubbardism) may be totally different from my pain in the zorch. Yet on the upper levels the Pre OT (Well, you can't very well call 'em a Pre-Clear after they're gone clear...) is given the EXACT specific incidents to run, as if everyone had the exact same experiences. Hint, no two people experience anything exactly the same...
2) The upper levels don't so much deal with Xenu, etc. as they do with waking up and kicking out body thetans (BT's). IMHO, this is the most dangerous part of scientology, as the rudely awakened and violently insane body thetans are dumped on the environment... Discussing upper levels is very tricky. She probably will pretty much refuse to talk about this part of the upper levels.
You can probably get her to tell you that as one goes up the bridge, one's abilities increase, at this point you could ask her if she's gotten more able to create effects in the physical universe, how does she know she's not creating the incidents she's being told to run in her own personal universe? Isn't she capable of doing that? Isn't that even easier than creating a parking place, or changing a red traffic light?
If you can get her to talk about BT's ask her if they are fully cleared? Where do they go? Now that they are wide awake, what do they do to other people the attach to? Ask her when BT's were first discovered, then quietly observe that the world seems to have been getting crazier since then...
Underlying philosophy bonus point:
If she offers to show the the emotional tone scale (One of the really useful nuggets in scientology) a measure of a person's well being and something of a map in how to relate to them from a higher position, and thereby bring them 'up tone' go for it!
Keep a copy for your own reference if you can.
Study it, talk it over with her. Then ask where is 'Love' on this chart? I see 'Hate', but I don't see love?...
Before she leaves re-emphasize that you love her and will ALWAYS be there for her no matter what.
Hope this helps.
The "zorch"? I thought that was from an old I Love Lucy -- one where Lucy pretends a plethora of symptoms (including amnesia) to persuade Ricky to let her be in a show. Ricky finds out it's all phony and calls an old show biz pal to pretend to be a doctor. He tells Lucy it's her zorch, which might have to come out if it's not too late. Really!
Is it true that eighties screen nerd Eddie Deezen is an OT?