Cats identify other cats by smell, the older cat thinks it is a new
cat.   (It smells diffferently).

I know cats get a little weird when one of them suddenly smells
different (like from a trip to the vet.) When we had an unweaned kitten
that needed to go to the vet, we took the whole litter and the mom in
case the kitten would be rejected by the others afterwards.

Six weeks seems like a long time to still be hissing, though. Have you
tried petting one then rubbing your hand all over the other? Try mixing
the smells on each other. It is probably just like having a new cat to them.

If there's no violence, just hissing and grouchiness, I'm sure they'll
work it out on their own. They're just taking their sweet time...

Good luck to all of you,

Rhonda