It can be done easily, depending on the hen... but that's a long time to be broody and they lose weight and condition during that time.

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Maybe not... but it is at least 25%- 30% longer depending on how you do your calculation. So to me, yes.Is 28 days that much worse than 21? In honestly not sure.
She got up so I snuck another egg into her nest. I'm thinking I'll let her try to hatch these and if they don't make it give her the ones I ordered? Do you think two eggs to sit in it good? I don't want to end up with too many chickens for the space we have. To recap, we have 3 BO hens, 1 silkie cockerel, and are getting more standard size pullets. Coop is 8x8 (we'll need to add roosting bars but that's fine) run is 8x16
In your situation, I would see if she wants to brood, but I would not give her eggs to hatch. I'd have her brood golf balls or ceramic eggs or something like that. The timing with her hatching her own chicks, then trying to graft shipped chicks may be too tricky. Lots of hens won't accept grafted chicks that are a few days older or younger than their hatch.
I'd give her the bait to set on (golf balls, ceramic eggs, etc) and watch her. It may take a few days to really stick to the nest. But if she is dedicated, it could work out nicely for you to graft the chicks to her.
If you do have her hatch out eggs, remember statistically half will be cockerels. You'll need a plan for them. If you just go with your shipped sexed pullets you still have a chance of an Oops, but it's much less than hatching odds.
As the younger ones get settled, remove the cardboard, and give them a wire divider. Then, start feeding them at the divider. Eventually, you can put one of the bigger ones in with the little ones during treat time. Keep rotating a big one in with the littles until you see how the bigs interact, then remove the divider for supervised play time. Integrating early is better than late. I've done 3 week age difference before without problem.We picked up some chicks that are about 2-weeks younger than the older ones. We put together a divided brooder in the hopes that the birds would gain familiarity before being integrated. (The little fuzzbutts are in the section on the right.) Nope. The small ones were terrified of the bigger chicks, and immediately ran to the opposite side of the brooder.
We added a cardboard partition to visually separate the sets. We plan to slowly back it off so they can get acquainted, but the little ones can still run away and hide in a sheltered area. I know they can hear each other, and both groups regularly peck at the cardboard divider. Haven't quite figured this one out yet, but a gradual approach seems appropriate.