Have those hens been in contact with any new chickens in the last month or more? Would you trust your sister to recognize a disease or parasite problem if she saw it and would she tell you if she did? It's possible they have essentially been in quarantine already. Does disease or parasites have anything to do with your sister's decision to get rid of her chickens?
It is not unusual for a flock to develop flock immunities. They can be carriers and infect other chickens but they themselves have developed an immunity. No matter how long you keep them in quarantine they will not likely show any symptoms. Coccidiosis is a great example but there are others. It is also possible yours are the ones with those flock immunities and they will infect the new ones If your chickens are coming from an auction or chicken swap quarantine is a great tool but here are some cases where quarantine doesn't do you much good.
There are different ways diseases or parasites can spread. Pecking at each other's poop, sharing food or water, insect vectors, or just on the wind. Your set-up does not offer much protection from disease or parasites but can be really helpful for integration. They will scratch stuff back and forth between the runs. The ideal quarantine is where even wind cannot carry disease back and forth or mosquitoes cannot go from one flock to the other. Like maybe at your sister's place. You should not feed or water using the same buckets or storage. Changing at least your shoes when going from one to the other will stop you from tracking stuff between flocks. A true quarantine is tough.
One way to protect against flock immunities is to pick a potentially sacrificial member of your current flock and house it with the new chickens during quarantine. See if any get sick. That might tell you which ones have the flock immunities. It is a good idea to check for mites, lice, or worms before they mingle too.
It is your decision, I can't make it for you. Hopefully this will help you make an informed decision. Good luck!
