analog

I'd say unless you're absolutely, totally, completely, unquestionably positive... like you can tell it's in your house, the girl has a tattoo or scar or something definitive that you can see that matches totally... it wasn't her. No matter how much you think it is... it isn't. Just gotta let the denial set in nice and cozy... it's not her. It's not her. It's not her.

That, and there's absolutely nothing whatsoever that can be done about it. At all. Just believe it's not her, and move along, and hopefully the memory will fade. Bringing it up to her can only cause problems, there's nothing to be gained.

Also: though you're her brother, and I know you want to, it's not your job to judge her or hold her accountable for her past- just to love her as your sister. I'm sure there are things we'd all rather we hadn't done, they just don't all land on the 'net in picture form. Just let it be, and you'll all be a lot happier.