She will most likely end up slipping so just expect her to. I wasn't going to tell my in laws until I told all my siblings but my fiance was just so excited and told them while we were out at dinner one night. I ended having to call all 4 of my siblings right after dinner because my mother in law posted it on Facebook minutes after we told her when we specifically said DO NOT say anything to anyone and don't put it on any social media...
Ask her how she would like it if some one else told every one for her instead of her being able to tell every one.
You could have a reveal party (not a Herbert reveal) and let her be a part of it, maybe she will be more willing to hold it in. Invite family and friends have a bbq or a dinner and at the end bring out a cake that says "I'm expecting or we are expecting"
Maybe then she will be more focused on the party and feel more like a secret. Im sure she's just very exited
My mother in law posted on Facebook as soon as we told her not to it was so frustrating. So since then we haven't told her anything until we were ready for it to be out there like the gender and we're not gonna tell her when she's born until after I've had a chance to tell people this time. There might not be a way to hush her this time but in the future try not to tell her until you're ready for it to be blasted.