Is there any way to make cats get along?

I know that cats have very strong personalities, and for the most part are independent and at least in my experience are pretty solitary unless raised together from kittens. However, I have a bit of a situation and was wondering if there was any chance that someone knows a way to soften cats up to each other.

Sorry for the long story ahead, just thought details might help.

So we have a very proud and spoilt 18 year old russian blue who was crazy energetic and excitable as a kitten, went through a mid life grumpy phase and has now mellowed out. We got a rescued moggy kitten when the russian blue was about 6 and they never got along. I don’t think the russian blue forgave us for a long time for bringing this peasant into her palace. Her introduction might have been the trigger for the russian blue’s grumpy phase. I would have to take a photo if they ever slept on the same bed as each other (though when they did at opposite corners mostly) because it was so rare.

A few years ago we moved into a house that has a laundry accessed from the outside, so the moggy basically has free reign of the laundry and backyard and barely comes inside anymore, while the russian blue guards the front yard and sleeps inside, and gets spoilt by my mother because of her old age and increasing fragility (the cat’s, not my mother’s).

3 years ago I moved interstate for university, and lived with my sister and her boyfriend. They bought a cat together from a shelter, but then they broke up and the boyfriend moved out. The cat stayed with me and my sister. Then we moved out to separate places and I ended up with the cat because my sister’s new housemate wasn’t keen. I was happy with this arrangement as I probably spent the most time with the cat anyway and I love her like my own.

But now, I am moving overseas for 12 months, and my sister has since moved back to our home state but into a house that won’t allow pets. I am currently back at my parents house until I get organised and have brought my sister’s cat back there with me until we can decide what to do with her. I would rather not give her away officially because I would love to be able to have her back when I return, and so the best case scenario is that she would stay at my parent’s house. BUT that is where the russian blue and moggy are, and if she doesn’t get along with them my mum’s saying no. I’ve kept my sister’s cat inside so she can get used to a new house so that when I let her outside for the first time she doesn’t just scarper, but when the russian blue comes inside to sleep they growl and hiss at each other. This is mostly instigated by my sister’s cat who hasn’t ever had to deal with another cat before, who arches and hisses at the russian blue, while the russian blue just seems to sit there like “what the hell how can you do this to me again you peasants” before making a pathetic whiny groan.

When I do let my sister’s cat outside, the same thing will probably happen with the moggy if she ventures into her backyard territory.

So, after all that, TL;DR, is there a way to get a stubborn old spoilt russian blue, a now very comfortable middle-aged moggy, and a young interloper moggy cat to get along?