First - are you and your fiancee moving into a new place together; that is, new for both of you?
If you are moving into her place, her cat will be the 'alpha' and Jasper the new kid.
1. Set up a separate room for Jasper - food, water, litter. Keep the door closed. Any sniffing can go on underneath the door as they get to know each other.
2. Switch beds/blankets that each cat sleeps on a lot, to get them used to each other's scent. Give Jasper lots of love...and after you cuddle her for a while, go to the other cat and do the same thing. AND vice versa....another scent exchange, but with nice scritchies and hugs!
I'd give this a week before letting them meet nose to nose...even then, be right there with them.
My Oscar got used to Cole in a week - but only because he was SO bonded to me, and I could assure him it was ok the first time they touched noses. His back fur went up just a BIT...and I patted him and told him it was ok. This was day 3. But still....they were only out together when I was home. After a full week, they were on their own. This is pretty rare, I think.
It takes as much time as it takes. Hisses and meowing are nothing to be worried about - yells, ears flattened back, baring teeth in a high growl that turns into one deep in the throat...um, THAT you worry about!
Good luck.
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