My situation with my sibs is very similar to yours.
I have finally reached the point with them you described above. I am not rewarding the behavior anymore. The behavoir where they act like I am sub human and make me cringe when I even think about being together.
Yes, that behavoir. Our last "time together" was a family event but inside myself it was a "goodbye to this crap" event.
That wont be happening anymore.
All that said, your current question about what to do with your sister gives me a moment to think because I am sure the same thing will come up for me eventually.
I think I will do whatever I would do if I wanted no regrets.
If your sister were to die and you regretted not sending the text message - then you should send it.
I can certainly say I understand your hesitation at knowing exactly what you want to do.
Isnt it sad to be placed in a quandary over a man made religion?
I hope it comforts you that so many of us here understand exactly what you are going back and forth in your mind about. xxxooo