Do Do you think broody Plymouth Rock (full-size) will be a good mom for bantams?

[Show Sebright]

How can I stop her from stealing the eggs for the other hen box? She had 9 eggs under her yesterday.

 

[50-45-1]

Unless you confine her, it is likely to continue. Other hens may also lay there eggs in her nest. Resulting in staggered batch. (not all eggs to hatch at the same time)
Not good.

 

[Chicalina]

She looks like she is in her own box. She cannot carry eggs from another box to hers. No opposable thumbs!

The other hens are laying in her box.

Have you marked the eggs you put under her?

I still recommend moving her to an area of her own or you'll be fishing out new eggs from under her every day. And that could prove risky if she gets too many and lets the original eggs get cold.