Would two sister hens (that have been with each other for a year) fight?

[dogandchickenmom]

They will be happier together, as long as Mary isn't causing more harm. Watch how they behave! If separate, have Martha fenced off in the coop so they are together but can't touch.
A game camera would be nice, if you have one, looking for the culprit.
Mary

That is a brilliant idea!! Thank you!!

 

[Skipper81]

Your chicken was attacked by predator other chicken got away.
Your injured chicken needs to be isolated until she is fully recovered. The reason your other chicken was pecking at her is because she injured and chickens pick on injured chickens.
Your chicken didn't rip out that many of her sisters feathers in one day and injure her eye. I've had chickens get into a fight or 2 and they don't do that. My guess is a cat

 

[chickens really]

Hello...They are both stressed so the one that got away is just treating the injured as all Chickens do, as the weakest link..keep them together and things will settle down..The less stress the better..Clean up the injured Bird and lock them in the Coop and Run for a few days..

 

[chickengeorgeto]

The pecking order is fluid and the balance of power can shift from one bird to another and back again as the birds interact with each other and their environment. So sure two full sister hens can definitely kill one another if they are so minded. However in your case I will speculate that your hens have endured a hawk attack. It is not normal for a hen who has challenged the pecking order to leave the field of battle once she has won a victory and the former alpha hen's back is uncharacteristically marked with some non-chicken looking damage.