But I never told her what really happened. I didn't want to make her choose between us. Okay, that's stupid, but I had all the wisdom of a 17-year-old. Meaning, not much.
Fast-forward, he now has dementia — and told her what he did, which includes pushing me against a wall with his hands around my neck, choking me because “you are evil” for graduating first in my class. It was my high school graduation. He kinda told her all of it. And I mean ALL. It's not good.
She's seriously upset with me — why didn't I tell her?? And she said, reasonably, what to do about his abuse was her choice, and I took that decision away from her.
I don’t know what to say to her — how do I repair a trust I broke?
Conflicted: Wait — where is her sympathy for a younger sibling who got choked, slammed and written off as evil? At 17? During what should have been their moment in the sun? Seriously.
Where is her heart.
You're not wrong about the points in favor of telling your sister. You did deny her a chance to make informed decisions, and you could have saved her the trouble of peacemaking efforts that were built on deep inaccuracies and therefore bound to fail.
And yes, you were 17 when you decided not to make her choose, but you didn't remain 17; you could have brought your acquired wisdom to the issue at any point since then.
But, wow. I hope you — both of you, but I assume I have only your attention — also give some thought and compassion to why you made the calculation you did. You didn’t approach your decision as your sister’s guardian or peer. It’s radiating from between the lines that you approached it as her inferior, and your father’s, too: the person who couldn’t help him as she could, who was “stupid,” who despite being a victim was still trying not to cause problems for others.
To the extent she is a victim here, your sister is a direct and secondary victim of your father’s behavior. Not yours.
Maybe your sister has already come around to that sympathetic view, or will soon; maybe she's just reacting right now and dumping her feelings on you because dumping them on Dad, where they belong, seems wrong when he's gravely ill.
But if she hasn’t gotten there, that doesn’t mean you have to bear an ounce of blame not rightly yours. “I should have told you, I see that now. I am sorry. I hope you can also understand his abuse conditioned me to disappear.” And does to this day. But please don’t: You matter, and you deserved better. Please let the truth set you free.