Any success stories on difficult introductions? I'd love to hear how you guys managed it!

*Background*

We used to have 2 6-year old cats who were litter mates. They got along fine, though the girl cat would occasionally growl at the male cat. She'd sometimes be chased by him but would end up later sleeping on each other.

Our male cat suddenly died, and a month later, we've adopted a 4 year old male from the shelter. We've kept him in a separate room for 7 days so he could acclimate. He's very playful and affectionate.

On the 8th day, we swapped rooms with the cats, so the girl cat could get used to the new cat's smells, and the new cat could explore the rest of the house. That went fine.

Later on we opened the door and let them see each other. There was immediate hissing/growling from the girl cat. I understand this is probably normal. I was giving the girl cat a lot of affection in front of the new cat, so it didn't feel threatened. That seemed to work. 

Every time the new cat moves around the room, the girl cat gets extremely aggressive. Even though i've prevented most of it. There's been a couple of times when the girl cat has chased the new cat and has made contact (lots of fur flying).

We've had the cats separated again so they can both relax. Now when I open the door, the girl cat comes in, and always looks to want to attack the new cat. 

The new cat, who was previously ambivalent to the other cat (even through growls), is now starting to show fear around the girl cat :(
user
Our older girl cat is growly and hissy, and wants to attack the new cat all the time. The
assistant
Any success stories on difficult introductions? I'd love to hear how you guys managed it!

*Background*

We used to have 2 6-year old cats who were litter mates. They got along fine, though the girl cat would occasionally growl at the male cat. She'd sometimes be chased by him but would end up later sleeping on each other.

Our male cat suddenly died, and a month later, we've adopted a 4 year old male from the shelter. We've kept him in a separate room for 7 days so he could acclimate. He's very playful and affectionate.

On the 8th day, we swapped rooms with the cats, so the girl cat could get used to the new cat's smells, and the new cat could explore the rest of the house. That went fine.

Later on we opened the door and let them see each other. There was immediate hissing/growling from the girl cat. I understand this is probably normal. I was giving the girl cat a lot of affection in front of the new cat, so it didn't feel threatened. That seemed to work. 

Every time the new cat moves around the room, the girl cat gets extremely aggressive. Even though i've prevented most of it. There's been a couple of times when the girl cat has chased the new cat and has made contact (lots of fur flying).

We've had the cats separated again so they can both relax. Now when I open the door, the girl cat comes in, and always looks to want to attack the new cat. 

The new cat, who was previously ambivalent to the other cat (even through growls), is now starting to show fear around the girl cat :(
user
New cat was initially hostile to the first cat. It started to calm down. Now the new cat is
assistant