Nice to see you Oz (always luved that avatar).
Reading too much into it? Nah. I think it would be worse than a flashback because it was tangible, like you were observing the setup for the abuse with the abuser right in front of you.
fwiw, I can't speak to your sister's possible reaction. Under the circumstances I think it's understandable a lot of fearful feelings about everything else would come along for the ride.
Ever talk with her about her memories of the abuse? That, to me, is a starting point. See how she responds. If positive, share a little more. If not - which really sucks because she's your sister, after all - you've an indication of how far you can go on disclosure.
Yeah, please go chill. You've just had a major shock and need time to recover.