Sounds a bit like she wanted this break. Just to test the other one.
She had probably already entered into an emotional relationship with the other guy before you started your break. And now she's seeing him without thinking about you. Go NC and move on. Block her above all and take care of yourself. Both body and soul. You don't want to know what she's up to from now on.
Start training very hot. This makes you feel happier and sleep better.
Start hobbies you didn't do before because of your ex.
Reach out to old friends you haven't been in touch with because of your ex.
This really could have been solved with a “you know we're twins and this is also my birthday. What the hell, dude?” when OP was told the plans.
Sounds a bit like she wanted this break. Just to test the other one.
She had probably already entered into an emotional relationship with the other guy before you started your break. And now she's seeing him without thinking about you. Go NC and move on. Block her above all and take care of yourself. Both body and soul. You don't want to know what she's up to from now on.
Start training very hot. This makes you feel happier and sleep better.
Start hobbies you didn't do before because of your ex.
Reach out to old friends you haven't been in touch with because of your ex.
Good luck!