Your jealousy toward your sister is certainly understandable given your ex-boyfriend’s attraction to her, but your intense jealousy is misplaced. Your ex is the one who hurt you, not your sister. I think you’re pushing your sister away to avoid getting hurt again, but it’s not going to work. Some guys will like you and others will like your sister. As long as you’re focused on keeping your sister away from all of your male friends you’ll stay stuck in the jealousy instead of learning how to move on in a healthy way after a relationship breakup.
Feeling hurt and betrayed after a breakup is normal, and the fact that your boyfriend broke up with you via text doesn’t say much about his character. When you feel the jealousy toward your sister coming up, identify it for what it is — hurt and fear. I think that the real question is why would you choose to get back together with a guy who told you he had feelings your sister? I recommend that you focus less on your sister and more on building your own feelings of self-worth so you can feel deserving of a relationship with someone who wants to be with you.
Take good care of yourself.