uhmm ok, I was smiling the whole time I read your narration.

But seriously, why do people do that?
For documentation?

I believe you are obligated to fellate them.

dont take photos then, theres always a moment when things come out.

Well... that's mostly funny, isn't it? Although it really is embarrassing, but years from now you can really look back at this and laugh. I recommend asking your sister whether or not she actually saw them so that you know what kind of story to tell at that time.

LOL. Not a big deal. Contrary to some advice here, I think the best option is if you just ask her whether she saw your naked pictures, and just in case apologize her for not warning her, stating it was not your intention for her to see them ever.

if you don't want others seeing your nude photos, don't post them on you da page or better yet, don't take any at all.

or maybe there's a way to password-protect the files on your computer??

(Assuming you are over 18)

Don't say anything. Awkward things happened, and part of life is learning to just go on. I'd imagine neither of you wants to have a conversation about this. Sometimes, it really is best to just ignore something and pretend it didn't happen. You don't even know for sure she saw anything, and if she's any kind of sane, she wouldn't have tried to.

Then password protect your stuff next time.

That sounds cute ^^
Can u send them to me xD

I have little sympathy for people who take nude photos of themselves in the first place. But unless they say something, just assume they have 't seen them. Maybe keep stuff like that on a usb or something other than the computer itself?

you could check the date last opened of the files

I wouldn't worry too much unless she brings it up to you, then deal with it head on.
Till then, just pretend you don't know anything. 

Unless your sister or other people (her friend, your parents) mention seeing your nude photos then assume that she either didn't see them or is polite enough to pretend she never saw them.  Other than that there's nothing you can do about something that has already happened so you might as well stop worrying about it.

To prevent it from happening in the future, put a damn password on your damn computer.  Then create a guest account for casual users like your sister and set the permissions for your nude photo folder to not be accessible to guests.  This way your family will be able to borrow your computer and your nude photos will remain safe from their eyeballs.

Don't keep nude pictures on your PC...?

Honestly, I wouldn't worry. They've probably done it at some point in their life too! 

So what if they saw you naked? Don't worry about it, they might tease you about it, but that will probably it.

As other people have pointed out, if you don't want certain people to see certain things you need to take measures to do that. You don't leave your front door unlocked when you leave the house and act surprised when you got robbed. As such, either put a password on your computer, learn to clear you browser history, or keep sensitive files in an external drive. The measures are countless, and now you learned the hard way.

In regards to your humiliation, is freaking out and being embarrassed about it going to change them seeing or not seeing anything? What purpose does that serve? You can't go back and change your mistake, but flipping out over it is a useless expenditure of your energy. Learn from your mistakes and move on.