Family Ties/Let her see the files.
Now that Heather as seen the files on your computer you figure there is no point in trying to stop her from looking at them and running the risk of pissing her off even more and causing her to scream at you, alerting the rest of the house to you indiscretion.
Heather goes straight for the file with her name on it and begins scanning the photos contained with in. Aside from some bikini pics, a few of her getting out of the shower and few more of her in her skimpy cheer outfit it's obvious that they were not at all what she expected. Most of them were of you and her holding each other, hugging and laughing, not at all like the pics in the other two files. You big sister confirms it by opening the one with the title mother on it finding several pages of photos with your mom topless and just in her panties.
She then goes back to her file and begins scanning the photos again. "Where..., what, where are all the naked pictures of me, you little pervert??" She grunts angrily. "These are all us just cuddling...."
"That's all that's in there.." You affirm in whisper
Heather glances back at you briefly and then back at the computer, tears dripping softly off her chin. With a final grunt she pushes the key board away and turns to face you as you prepare for a well deserved tongue lashing, but it never comes. Instead Heather practically explodes into a river of tears, covers her face with both hands and rushes past you out of your room.