What a tough one. I think you need to be just be up front and tell her and maybe tell her before you tell anybody else. You certainly don't want her to hear it on the grapevine and I know I wouldn't want to be the last to know in your family/friends group. I know that it will be difficult for her, but I am sure being your sis she will be equally happy. I really don't see you avoiding hurting her feelings (sorry) cos it is such an emotional rollercoaster ride ttc.
Maybe you and her can get together for a girly day and 'break' it to her that way.
Good luck.