Whether she be an older member, or brand new to the house, the sting is still there. I have three words for you: Move on, honey.
If your ex is part of a fraternity on campus, you can’t be too surprised. He clearly liked you for your personality, and the way you present yourself. So when you break up he’s going to be attracted to someone who has those same characteristics. And who is more like you than your sister? You share the same values and have a passion for the same philanthropy, its no wonder why he is interested.
Do your best to not even be around them. If seeing them together at formal or a date party will be too much for you, keep your distance and be respectful towards your sister. Even if this really upsets you, she might be in the best relationship of her life and its not your place to ruin because you’re jealous.
Take this anger/hurt and put it towards something great. The best way to get over something is to get it off your mind! Pick up some extra community hours (you can never have too much), or start going to a work out class with some of your sisters. These activities will boost your self confidence and make you wonder why you were so upset in the first place. Don’t just sit around and think about his new relationship, the thoughts alone will drive you crazy.
When all else fails and being miss nice girl isn’t really your thing, make him regret his actions. You don’t need to “win” him back or disrespect your sister but there is nothing wring with showing him what he’s missing. Is he going to formal with her next month? Put on your best dress, get the hottest date, and dance like he isn’t watching (even though we all know he totally is).