The hiss at the front door is baffling if you expect the cats to recognise each other on sight. Cats run their social world largely on scent, and a cat home from boarding carries the smell of a different building, unfamiliar cats, disinfectant and strangers' hands. To the cat who stayed home, the arrival in the carrier looks like her companion and smells like an intruder, and she responds to the smell. That framing matters, because a scent-identity problem has a practical fix, while a supposed feud invites despair.
Step One: Brief Separation
Begin with brief separation as soon as the reaction shows. Set the returning cat up in one room with her own tray, water, food and bedding, and let the resident cat keep the rest of her territory. Separation is not punishment; it gives both cats time to calm down, lets the cattery smells start fading from the returning cat's coat, and stops a bad first hour hardening into a pattern of ambushes and chases. Groom the returning cat gently with a soft cloth if she tolerates it, and otherwise simply give her time in her own home's air.
Steps Two and Three: Scent Swap, Then Feeding at the Door
Next comes the scent swap with home bedding. Rub a cloth on the resident cat's cheeks and chin, where her scent glands are, and stroke it over the returning cat; do the reverse with a second cloth. Swap their bedding too, so each cat sleeps on the other's smell, and rotate which cat has which room for short periods if both stay calm. You are rebuilding a shared household scent, and it cannot be hurried.
Then move to feeding either side of a closed door. Meals are the strongest daily reason to feel good near a smell, so feed the cats on opposite sides of the connecting door, far enough apart that both eat relaxed, moving the bowls gradually closer across days. Hissing at the door means the bowls are too close; eating calmly means the association is working. A treat or a play session near the door for the resident cat adds to the same lesson.
Step Four: Short Supervised Meetings
Only after door feeding is calm should you try short supervised meetings. Open the door for a brief session with an easy exit for both cats, feed or play at a distance, and end the meeting before either cat has time to fixate or escalate. Several short, boring meetings rebuild familiarity far better than one long tense one, and a step back to the previous stage for a day is progress, not failure, if a meeting goes badly. Most pairs recover fully over days; if aggression persists for weeks, or either cat stops eating or hides constantly, your vet can advise on next steps.
Set up Care Pass through KinsLe before any future stay, naming the particular carer who will look after her, with access limited to the dates away.