You've done well to keep it to yourself as it is!!!!!
If you are close to your sis then just be frank with her and let her know you're expecting as you wanted her to know first and not second hand, but for her not to tell anyone else, she may then feel exclusive to your news, and really chuffed that you have confided in her, being the way your parents react.
My sis-in-law told me she was expecting 2 days after I started to have a mc. She wasn't happy about her pregnancy at first, so she was crying etc and it all became about her, and not me and the mc I was actually suffering. I am not that close to her so I just felt she was inconsiderate, and the more I think about it, the more I think it was damn right awful to do that to me at that time.
So it really depends on how your relationship is with her....and how you tell her.....
Remember both your hormones are all over the place!!!!!
