Future announcing

I don't even know if I am pregnant or not. But when the time comes I only want to announce it to our parents and siblings. The issue I'm having is that I am terrified of telling my husband's sister. A few years ago she took to fb and announced another family members pregnancy even after her mother told her not to due to the fact that not all of the immediate family had been told yet. I want to wait for extended family and a social media announcement until I hit my second trimester due to having a miscarriage back in February. So I am looking for advice on how to handle this situation when it comes up. I want her to know but hate that she has a big mouth.