Dating other men is not the answer to your problems, even if it's your motive to make jealous. Make him aware that you're very serious that you want him back, and that he's special. Again, jealousy won't help him notice you care for him by any means. That would make me want to never to talk to you again, especially if I cared for you. I'm the type of person that (out of respect for the new man) will quit talking to an ex, because I know I would be upset if my new girl kept talking to her ex. Maybe that's how he thinks about the situation.
About your sister, I don't know what to say about that. It would make me think something was up, but if your sister is a decent person, she would never do anything either way, so I wouldn't worry about it. I'm sure it was a harmless.
All and all, I would just keep to myself for awhile, and quit going out with other guys. It's not fair to them if they actually become interested with you either. That's just not right.