Yeah...about that. >.>
That's the latest rookie mistake I've made. I had hens brooding eggs but I didn't isolate them from the rest of the chooks - so for about a week afterward, chickens kept climbing in and laying fertile eggs on top of my brooding hens. So the spread of hatching dates is over about 10 days. :/
That happens to all of us . So had this egg pipped externally before momma decided to help it along ? If so, we can assume that it should hatch over the next 12-24 hours. Being as you have a staggered hatch I would get that brooder set up anyway. Chances are momma will up and leave the others once she's had enough.