He obviously misses his sister and his familiar environment. Does he
get a chance to go out and socialize? Sounds like he's lonely (for cat
companionship) and cooped up. My cat is completely silent but finds his
voice and meows loudly all day when I'm away on  holiday (or a visit of
more than a day). Even if my family is home to take care of him. So you
see! They hate change.
 One other thing. I know you must neuter him but do wait till he's
settled down. It'll only add to his trauma now. And I'm personally
against declawing. I just trim the tips of his claws very carefully,
avoiding the pink part.

Neutering should be done as a matter of course but why would ripping his
fingers off quieten him down? How can you think such a thing? That's
terrible.

Siamese are well-knwn for their vocalization, so it's possible that at least
part of what you are hearing is simply a result of heredity.  Nevertheless,
you should have him checked by a vet to make sure there are no medical
problems.  Make arrangements to have him neutered at the same time.  This
really may help with your "problem."  It also should be done ASAP because an
unaltered male may start spraying.  Believe me, you would *not* like that
scent, and it is easier to prevent spraying by neutering (not 100 percent,
but very high) than it is to stop a cat from spraying once it has started
(much more problematic).

Please DO NOT have your cat declawed.  It is cruel and unnecessary --  
declawing is actually amputation, not just "trimming."  Ironically, you are
also likely to cause still more problems by declawing because a much higher
percentage of declawed cats develop behavioral problems (biting, scratching,
etc.) and medical problems (such as early onset of arthritis).

MaryL

He's lonely. He doesn't like the restriction in his living space and he
misses his sister.
Wouldn't you?

He needs to be neutered, firstly. He must NOT be declawed, secondly.
That is an extremely cruel and confusing experience for cats and it
will inevitably worsen any negative behavior patterns.

Since his life has changed alot, you need to work a little at making
the new environment fun for him, with a new routine that he can get
used to. Does he have a cat tree? If not, get him one - no matter how
small the apartment - there's always a way to put a tree up near a
window. Institute regular play time and look for new and interesting
cat toys to incorporate. (Panic mouses, thing in a bag etc). Experiment
with treats to find things he likes and have an evening session of play
followed by hand-feeding some yummy treats. Set up some soft places
around the apartment where he can sleep and hang out when you're not at
home. If he's smart and vocal - you could always buy a cat harness
leash (ties around the chest) and take him out of the apartment to the
building lobby or courtyard on a leash for a walkie.

Try to take his needs into account and give him some extra support
whiel he's getting used to what has happened to him. He is complaining,
that's all. Try to show him that you are listening and that you hear
him. We all yell louder when it seems like no one is responding :>

He loves you and he doesn't want to be homeless. He just needs a little
help adjusting.

Best,

Tracy

Sounds like the Siamese in him. Siamese like company. His sister was
some company, but since she's gone, you're it. Another cat now might
help or make it worse.

They also have LOUD voices. And they talk, sometimes endlessly. I dont
know if neutering wd help, though spaying helps with a female. (Females
go into heat and yowl loudly and continuously until you need new
neighbors. Dont think males do this.)

Gets better over the years.