I would say you should definitley tell the one sister that her input is really important to you, that you care about her feelings etc. etc. but that the majority rules in this case. It will be tought, but she has to understand this is a mutual thing, you can't just say "I want YOU" to a sorority, so going to meet with the ones that are intersted seems like your best and only chance at this point. Tell the girl that either they get A sorority that is interested in you guys, or you wait and wait and hope a different one is, by then it is too late and you might risk it for the whole sorority....either way GOOD LUCK!! You are doing great...
