..and that makes it better? I dunno about you, but for me if I have something to confess, I'd do it to people I am very tight with, their opinion means the most to me, and because they know me as a person they can help me better than those who don't. ...but then again it depends on the concerns ¯\_(ツ)_/¯
Naomi just talk to your sister. If you don't want drama to get into your closer group of friends, which you said your sister is part of, it's best to solve these things immediately. If you can't both get the girl and keep the normal relationship with the rest, then choose the one you want more. You don't need a guy with a Hiroomi avi telling you which one to take. Simply depending on how much you like her or if it's just an affect, you should be able to weight both things on a scale and decide yourself.