Wow, at least people are being honest on this thread!
Compared to what went on in the thread where the woman asked if she should become a prostitute, and several people jumped down her throat... (even though half of them would have liked to see a video of her activities, I'm sure), I'm glad to see none of that going on here.
People generally choose to be photographed (unless it's hidden camera stuff), and yep, in this day and age, you're likely to show up on the internet at some point. That said, I agree with Jinn. Unless you're absolutely sure that your sister did it for money and she wanted it to be posted on the internet (which you don't seem to know, for good reason!), I don't see any harm in sending an anonymous e-mail type thing. If she doesn't care and did it in full knowledge of what would happen, she'll ignore the e-mail. If she didn't know, then she can deal with it on her own. Either way, your conscience is relieved.
Now, about sharing it with other people on this board... haha, that's up to your personal ethics, I suppose!