That's a lovely picture of her. Whenever we lose a pet and the others are effected, I give more treats. It seems to be a good distraction and cheers them up a bit. Depending on the species, car rides, toys or walks work, too. For chickens, I'm thinking snacks. It makes me feel better to give them treats, too.
I know it's hard not to worry, after you've just lost one of your girls. Occasionally, chickens just pass away, without any warning or symptoms, without it being from any type of communicable disease, even young ones. I don't know if they just don't have a good heart or what it is. Diseases usually have symptoms that you can see.
I know you need to keep an eye on them, but try not to worry too much.