Of course man! . . . for her f***ing fault you're in that nasty situation. I would be angry too if that happen to me. You must get revenge! She did an awful thing to you . . . you can't let that pass! She won't learn a thing as a human being!
First, take some audio recording of your girlfriend confessing what she did to you . . . and also you should get an audio recording of her stupid brother blackmailing you. Do it carefully!
Second, you say "Okey honey, I forgive you this time" and start to do a normal relation with your girl. For a few weeks or months . . . f*** really good with her. Take your girl out, spend a weekend in some place only you two. Do lovely things for her . . . and somehow, try to convince her that she has to recover or delete those photos (maybe she won't do it, maybe she will do it . . . by love). Man . . . you have to consider the fact that those photos could already be in ciber-space. You can't trust her brother, you can't trust your girl!
Third, tell her in a convincing way (after you have sex) that been tied up and gagged wasn't so awful (you have to search about "bondage" and "knots" in the Internet). Then, propose to tie her up just a little (with a few scarfs) and give her pleasure (could be oral sex). Repeat this inconstantly when you have sex with her . . . and every time increase the experience just a little. Make this girl enjoy it!
Fourth, after gaining her trust, you’ll need to find another opportunity to be only with this lady . . . a weekend in some place. Now is time for sex! You have to tie her up to the bed . . . do it softly but really really tied. Must be inescapable. Use rope. You also need some used boxers (with a little nasty smell) . . . and put them in her mouth (watch your fingers or hands, she could bite you) . . . and then seal it well . . . with a lot of duct-tape.
Finally, you take photos or make videos of her with a camera. Tell her what you really feel of the previous incident . . . how you felt betrayed. After that . . . untie her (you must do it!) and break up with her gently. You leave the place in that very moment with the photos and videos.
-If you do it, you must be always in control of the situation. Don't make yourself an accidental killer or something.
-If nothing of this works, take some legal actions anyway. You deserve respect, so respect yourself!