Don't bother your head, find yourself another guy and move on.
get over it
If she was real, she will not become an ex.
You do absolutely nothing about your ex boyfriend considering he has a new girlfriend. If he loved you he would break up with his girlfriend and since he has not then he wants to be with her and you need to accept that. Just because you have broken up with your boyfriend it doesn't mean you can just step back into your ex boyfriend's life.
maybe he wants you back.
Simple Answer: You don't.
Let him have a threesome with your sister (only if hot)
You guys are not meant to be
No, you should not email your ex girlfriend to try and get her back because she has another boyfriend. You had your chance to make the relationship work out with her and since you didn't then it time for both of you to go your separate ways.
If he has a girlfriend, don't go back. He will do the exact same thing to you, that he will have to do to his current girlfriend. Good Luck!
It means the ex boyfriend still likes the ex girlfriend.
Give up. She has moved on. It is time for you to do so as well.
If you are indeed keen on getting your ex girlfriend back, even if her ex boyfriend want her back, what you can do to make sure that he will choose you is make her see your best foot forward. Show her how better you are over her ex, how happy you can become if she decided to come back to you and how things will be better now.