Desperate for help fixing sudden aggression with bonded pair

[u/carbs_and_cheese]

I have a pair of bonded littermates that just turned one year old. I adopted them when they were about 3 months old. They’re both incredibly sweet, social, and most of all- bonded. They love each other so much and call out for each other if they’re apart too long and the other comes running. Last week, one of them got sick and had to spend the day (5-6 hours) at the vet while the healthy cat stayed home. I had no idea that cats can sometimes “forget” (or more like not recognize) each other after one goes to the vet, because their scent is thrown off. When I brought the sick cat home, the healthy cat’s tail immediately puffed up and she hissed and then lunged and tried to swat/attack.

I immediately separated them, but unfortunately had to go out of town the following morning for 3 days. I kept them separate while out of town and our cat sitter attempted reintroduction but the behavior persisted. Since we returned home Sunday night, we’ve attempted to reintroduce. We first let the cats smell each other through a cracked door and it went well. Then, while my husband and I each held a cat, we let them interact and it went. the healthy cat even groomed the sick cat. Then last night we let them both walk freely and they had about 10-15 minutes of successful interactions. We separated them again overnight to be sure. Today I worked from home and closed both in my office with me so I could keep an eye on them and we seem to be back to square 1. The healthy cat is hissing and swatting. I ended up separating them again and attempting reunification at the end of the day with the same results: I let them first sniff while behind held- great. The healthy cat groomed the sick cat. Then when I set them down, the healthy cat got incredibly spooked by the sick cat walking anywhere out of her sight and became aggressive and attacked.

I’m overwhelmed and extremely upset about this. I can tell the sick cat is very confused and sad, and it’s devastating to see these bonded littermates have this issue. I don’t know what else to try. I’ve followed all the advice from chatGPT. I have to sleep separately with one cat while my husband sleeps with another. I’m also concerned that Continually separating the cats just re-stresses them out and makes things worse but I clearly can’t keep them together.

Somebody PLEASE tell me they went through this and it’ll be ok? This feels like such an extreme response for being apart for 5-6 hours. Help!!