Take heart. The situation is actually not too bad. If all that happens
is a little batting and hissing with no injuries or fights, you're doing
better than a lot of the people who come here for advice on this.

My advice, FWIW: Give them some "alone time". For some time each day,
Marigold goes in the other room with the door closed and Whiskers gets
your undivided attention for play, laptime, grooming, or whatever. Then,
for another time, Marigold gets the same time with you, no competition.
Any time EITHER cat initiates a "hissy fit" - whoever "started it", so
to speak - the instigator gets put in another room for a time out.
Immediately, so they can connect the behavior with the consequence. A
firm 'NO' as the hissing occurs can add to the correction. Say NO
firmly, but not loudly, and (go) get the kitty and put them in the other
room for 5-10 minutes. The consequence of being a brat (hissing/batting)
is that you take your attention away.

Don't forget to praise good behavior! Treats and extra pets for Whiskers
for NOT hissing can give added incentive to "be nice".

If it makes you feel better, it took almost a year for my new addition
(Jeffrey) to be fully accepted by my Rowan. Time outs were very helpful.
They still are, actually, when Rowan decides to be a brat. *LOL*

Well, when you put it that way.

And my solution also comes from THANKSGIVING. What a coincidence.

I went to some friends. I took a laser pointer along, mostly by
accident. Had been playing with my cat. Nope, I took it along
to play with their cats if I saw them.

They were really, REALLY enthusiastic about the laser pointer.

WHY?

It solved the problem of their older cat getting along with
the new kitten. JUST LIKE YOU.

Both cats actually chased the pointer together. So this was
togetherness, if you will.

Just had to pass this along. Will it help? I don't know.

I do know the little laser pointers with the buttons are a big
pain. The buttons are more expensive usually than AAA's
and harder to buy.

I found AAA pointers at Best Buy for $14, good deal. Nice and
bright.
Try not to shine it in the eyes. Humans have a blink response
and not sure about cats. They also avoid direct exposure.
But the pointers you buy here in USA, if that is where you are,
will have a DANGER thingee on them. So don't shine them in
anybody's eyes, as a matter of course.

OK, this may sound insane, but bear with me.  I was in the *exact* same
position -- have an older cat, got a younger cat, younger cat was wearing out
older cat with his antics.  By chance I happened to take in another young-ish
cat, and.... problem solved!  The two youngsters now keep eachother entertained,
and old girl is left alone.  It's not much harder taking care of three than two,
and now when old girl passes on, the two youngsters will still have eachother as
pals.

M9