Darky, how awful.
What about if instead of agonizing over whether you can believe her, you try to make your judgment only based on what experts say the evidence shows (hopefully, she was examined, in which case they can tell forensicallly what is consensual and what was rape). It's completely understandable that you are tortured about not being able to trust her, with her history of damaging herself. So don't attack yourself for your doubts.
She could be a rape victim or not a rape victim, but in either case she is mentally ill. (And mentally ill people can get raped.)
She is horribly wounded either way and what a desperate time you've had trying to deal with it.
I am very sorry. I think it's really important to understand that YOU ARE NOT A BAD PERSON BECAUSE YOU DOUBT HER. Mental illness and severe instability DO cause others to doubt, and even if that proves incorrect, there is nothing bad about you for having approached it with doubt at first.
What an understandable response that would be.
I think you urgently need some help in getting more emotional detachment from her. That does not mean no longer loving her. It means loving her with more detachment. And you need help to do that, I think.
You sound kind of enmeshed with her and her troubles are dragging you down too.
For her sake, especially your children's, and your own...I think you need to make a decision that it's not going to be high drama love for you any more. It may be the sad love of a grownup for an abused child. There is tragedy here, but you can extract yourself enough to find sanity.
You would not prove your love, nor prove what a good sister is, by joining her in despair and irrationality. If she has been raped, we need a grownup around. If she has not been raped, we need a grownup around.
And you need help, to save YOURSELF from being permanently infected by her pain and chaos.
You have have profound love and compassion for someone WITHOUT JOINING THEM IN THEIR VIEW OF LIFE. You can have profound love and compassion for someone WITHOUT ACCEPTING THEIR STORY AS YOUR OWN STORY.
I think you need your own story, Darky. I think you need urgently to find some regular source of counseling and support.
Can you? What resources can you turn to so you're not trying to cope with all this alone? If you get involved, deeply involved, with a process of healing yourself, you'll be better off. And so will she. And so will her children.
Don't go crazy too.
Hopalong