She doesn't necessarily need to give up being with your cousin in order to make amends and rebuild trust. Plenty of poly people stay with the people they cheated with, while they worked on the relationship with the partners they cheated on. Forgiveness means to wipe the slate clean as if it never happened. You don't sound like you've forgiven her at all - in fact you sound quite bitter and vindictive. You're punishing her and trying to control her, but aren't getting any satisfaction out of that because to do so is really futile. You're having a tantrum. If I were either your girlfriend or your wife, I'd dump you for being so immature. I recommend counseling with a poly-friendly therapist, and stop thinking you have a right to punish your wife. Move on!
The world opens up... when you do.
Oh, oh, can't you see? Love is the drug for me. ~Bryan Ferry
"Love is that condition in which another person's happiness is essential to your own." ~Robert Heinlein