A story of how it could work when one of your sister's saved the day. This also involves a PiKA. Freshman year, the guy I was going out with (not steadily) was non-Greek and didn't get why anyone would want to be. He was fun, but I don't think he owned a suit, and given that he sometimes wanted to go dutch at McDonalds, I knew he'd balk at any costs associated with formal. Bottom line, he was not a candidate for formal.
A couple of weeks before formal, I was out with friends and we stopped by the PiKA house. I met a brother (a sophomore) named Rob and we talked non-stop for an hour. When my friends were ready to leave, Rob said, "Well, I'm really sorry this great conversation has to end." I was so hoping he'd ask for my number, but he didn't.

A few days later, I'm in the Phi Mu suite lamenting my dateless status when a pledge sister said, "You and Rob were hitting it off at the Pike house -- ask him."
A group of older actives were nearby. Three were seniors and engaged to Pikes. I saw a glance pass between them and one said, "Would you like to go to formal with Rob?" I said I'd love to, but given we'd only talked for an hour, wouldn't he find that weird or desperate? She smiled, explained fraternity men knew how formals went and not to worry about that. The next day, she called me and said, "Rob would love to go to formal with you -- but he does want you to call and invite him yourself." Reasonable. I called, he accepted. There was still a week or so before formal, so Rob took me out -- dinner, movie -- just so it wouldn't feel so awkward. Ah, there was eloquence to those days. We had a great time at formal, too!
I had a sister whose boyfriend broke up with her a couple days before formal. She was very upset, but still wanted to go to formal because it was her senior year. Some calls were made an a fine southern gentleman of Kappa Alpha (who she'd never met) was happy to escort her. He's still escorting her -- they got married a few years later!