LisAlly, seriously don't worry about it. I don't know how it is at other schools, but at my school you can't be "released" from COBing. You either get a bid or you don't. If you were the only PNM it is entirely possible, IMO, that the sisters were just like "Okay well since there's only one girl here, we don't need the extra time to get to know multiple girls." I'm sure they appreciated that you had the interest to show up! That interest seems like it would only be highlighted by being the only PNM in attendance.
I hope everything works out the way you want it to! Please let us know... I'm rooting for you!