Both our cats are from same litter and are brother & sister of 8 years. The
past 8 years they have been very loving and affectionate to each other, only
with the odd paw fight with a bit of hissing. They are both primarily indoor
cats but wonder in our garden when supervised. Two weeks ago at about 0230
am we were awoken to the most horrendous noise from both our cats, hissing,
hackles up etc...

The female cat would not let her brother near her and he
was getting very distressed which made matters even worse. We found urine
all over our dining room floor, not sure who's, hence a sleepless night for
all with both cats very distressed. The following day we took the female cat
to the vets as she seemed more distressed than him. That morning before she
went to the vets he was acting normal but she would still not let him near
her without a very loud hiss and growl. We found out from vet she has
arthritis down her back and one hind leg, she is now on permanent
medication.

Everything seemed OK for two weeks with everything fine between
them as if nothing had happened until yesterday when she was in the garden
with my wife and he was in his basket by the door. All of a sudden he came
charging out hackles up to his sister who immediately hissed etc.. and it
has started all over again.

The only solution we can think for this
behaviour is there are other cats that live in this area and maybe there was
a scent in the garden and being that is was windy in the garden could this
of provoked him to be like this.

Both of the cats have been spade and neutered since kittens....

Any help would be appreciated as we are very worried owners.