15 sounds like a few too many...I had a standard sized hen sitting on 9 eggs. I standards can sit on 10 or even a few more. I don't know if your breed are banties? I would think the number would be lower.
Here's what I did on my first trial with a broody.
Prepared a broody nest in a separate coop that was very close to the ground (necessary for any tumbling chicks to be able to get back under mama if they fall out of the nest before the others hatch). Provided food and water.
Carefully moved the eggs into the new nest.
Brought mama into the new space. She checked things out a bit and then settled right down on the nest.
She was rather stubborn....didn't want to leave her eggs for nothing. I gently lifted her off the nest and placed her in front of the food/water if she didn't do so herself at least once a day. She even poo'd in the nest once, but missed the eggs and I cleaned it out without disturbing the eggs.
Just let her set. Before I knew it, there were little fuzzy butts in the nest!
SO easy!
I would suggest you separate, as the other hen may try to lay in the same nest, possibly crushing the eggs. Or, mama may set on a different nest....
I don't know what she would do with the 5 week old chicks??
There are a few websites/blogs out there that offer great info on caring for a broody hen. Ofcourse, do a search here first!
HAVE FUN!