broody bantam hatching 6 eggs, one with crack

[Catus]

So, my young bantam pullet got broody and decided to hatch some eggs, despite the fact that it is autumn and today even snow came down. I live in Northern Europe and in the next 2 months the temperatures will be probably around 0 degrees Celsius.
She has hatched the eggs about 10 days now and I have candled the eggs a few times: six eggs, all seems to be fertile and show some development, although seems to be a bit delayed when I compare to the pics on Internet. My candling lamp is not very good and I really can't see details (e.g. blood vessels), but I can see a dark mass (about 1/5-1/6 of egg size) inside the egg and the air sac is much bigger than in not developed eggs in my fridge.
Can the eggs develop slower due to cold weather (coop is windproof, but not insulated)? The pullet/hen covers all the 6 eggs and seems to keep them warm, although 4 eggs are actually normal large eggs, not hers.
Yesterday I found that one egg had tiny crack, but it is still intact and today it looks the same as yesterday. It probably came from the hen that is currently molting and she probably just laid egg with fragile shell.
Should I remove this egg?

 

[GingersHuman]

I've heard you can seal the crack with candle wax, but I have no experience in this.

 

[JanetMarie]

Since the hen covers the eggs they will not develop slower due to cooler temperatures. If you would like to keep the cracked egg, you could try to repair it with superglue, otherwise remove it so it does not start leaking.