If I saw Michelle's post on Facebook, I would think that Jeff and Allie had given her the go-ahead to post about it.
If I then saw Jeff and Allie's own big post a few days later, I might think, "Hmm, I wonder if Michelle jumped the gun."
So I think posting their own announcement would be win-win for Jeff and Allie. In fact, I would suggest they make their own announcement super-nice, with details and photos (as they feel comfortable with)--reduce Michelle to the opening act, and have their own moment.
And after that, I would make sure I didn't tell Michelle any news of importance, until I was ready to have it posted on Facebook, or had already done so myself. Since Neanderthal days there have been people who couldn't keep their mouth shut about news; if that irritates people, they learn to tell the secret-spillers (and anyone who might spill to them) last.