Very nervous to tell my soon-to-be sister-in-law...?

So, my fiance's sister got engaged in February, we just got engaged. They are getting married in Sept. 2012, we wanted to also but are respecting her feelings and getting married in July 2012. We haven't announced our engagement yet and i'm really nervous to tell his sister. I talked to his other sister about it and she told me it was "her year" and that we should wait until 2013. That's 2 years away!! We don't feel we should have to put off our lives just because she is "claiming the whole year". His sister and I are pretty close so I don't want this to affect our relationship. We have been together for over 5 years now, they have been together for 1.5 years. (Which doesn't really matter) But I feel if it was the other way around... they wouldn't wait to get married... Any suggestions??