So my question is, will a "sitting" hen allow others to lay in her nest,
Yes. Occasionally you get a broody that may keep other hens out but most of mine are quite happy to let another hen lay in her nest. Some broody hens will even move an egg from another nest to hers. Rare but it happens.
and will she sit other hens eggs?
A broody hen will try to hatch door knobs, golf balls, turkey or duck eggs, and certainly other hens' eggs. If there is nothing there they can mistake for an egg they will often imagine an egg is there.
Just wanted to know if she is actually trying to hatch these eggs or if I'm just chasing unicorns.
And what's wrong with chasing unicorns? You may never catch one but who knows what will happen during the adventure. I like that phrase, chasing unicorns.
My test if a hen is truly broody and deserves hatching eggs is that she has to spend two consecutive nights on the nest instead of her usually roosting spot. There are a lot of signs that a hen might be broody or thinking about being broody, but those don't always mean she really is.
If she is truly broody, if you have fertile eggs, and you want to hatch eggs, I suggest you collect all the eggs you want her to hatch and start them all at the same time. Remove any other eggs under her when you start them. Mark those new eggs and check under her daily after the others have laid to remove any that don't belong. As long as you remove them daily you can still use them. That's the way I do it.
Others, for their own reasons, isolate a broody during incubation. Fix a predator proof pen that contains a nest, food, water, and a little room but not much for her to go poop. Lock her in there so she cannot get out and no other chicken can get in. Move her at night with a few sacrificial eggs to see if she accepts the move. Many do, some don't. Once you are sure she has accepted the move give her fresh eggs, all started at the same time.
The reason I want all the eggs started at the same time is that you don't want a staggered hatch, especially your first time. The hen has to decide if she will take the first to hatch off the nest to find food and water or wait for later eggs to hatch. Usually she abandons the unhatched eggs which may be developing. Staggered hatches can be stressful for you and are often not all that successful.
Other than the issues of a staggered hatch, the reason i want all eggs removed daily (or her locked up where she cannot get new eggs) is that if the number of eggs build up so much that he cannot cover them all, one gets pushed out from under her, gets cold, and dies. Then that egg gets back under her and another gets pushed out to die. If she cannot cover them all you often get really bad hatches.
Good luck!
