Reacquainting two cats

[tootiredtobeinspired]

We have 3 cats, an 18 year old girl, a 5 year old girl and a 3 year old boy (all neutered). Our 5 and 3 year old have never been hugely keen on each other but would tolerate one another. However this summer the 5 year old decided to basically move out and live next door (elderly neighbours with no pets who were delighted that she was coming through their windows at night!) Anyway we tried a few times to make her come home but she was having none of it and anyway she had somewhere else to go. In the meantime her absence meant that when our boy cat did see her he saw her as a stranger and chased her off! Problem is our neighbours have now decided they don't want a cat (they're off on holiday and also have no cat flap now the weather is changing) so we've had to go and collect the 5 year old girl in her carrier and bring her home. So far I've kept her separate from our boy cat but they are both totally freaked out by the strange smell. Neither of them has any problem with our old girl but they seem to hate each other! What do I do to make them tolerate each other again??

[Toddlerteaplease]

Could you try some scent transfer? Or feliway friends. I used that when I introduced a new cat and it went very well.