We had an incident where one cat got stuck in the cupboard and she knocked a box over when she came out. This spooked the other cat and this sparked up aggression, particularly hissing, growling, yowling etc.
It's been 18 days and we've gone from day one where the cat would hiss at anything with her sister's scent on, to now day 18 where they can sleep in the same room together, sometimes pass each other in the corridor without hissing and take treats pretty much nose to nose. The 'aggressor' has groomed her sister a little but she still hisses at her sister after grooming her for a bit, there are still somewhat frequent bouts of hissing which we interrupt. She's clearly still not comfortable, which we are hoping with persistence we can get her at least comfortable again.
Just want some reassurance - will these two cats ever return to their normal selves? They used to nap together, groom each other and eat happily together, I can't see me being able to leave them unsupervised again, ever.

Thanks.