Honestly... who the fuck GIVES a shit if you dump your current girlfriend ? You don't care enough for her when compared to the passion you feel for her sister.

I say be honest, if you hold on to honesty you will know that even if things went wrong or even if you are sad and ronery well... It's what you desired, in a way. Don't be afraid to be hurt or hurt people, for that matter, everyone heals and you should just live your life to it's fullest, take chances and be happy !