[fyrnymph]

I don't understand the "honorary sister" thing. Aren't you either a sister, or not? Regardless, there's not much you can do about that without looking petty.
It's tough, but count your blessings - you found out their true natures before you invested any more time in them. I don't think you should worry about either of them. They both have demonstrated they don't give you the consideration you deserve. If you can work to a place where you can forgive them, good. But that doesn't mean you have to associate with them, or even like them.
Best of luck - I've had friends turn out to be real backstabbers. It's no fun, but I hope your good friends and sisters can support you.