Time and love and maybe giving Ruby some time out in a room for a couple of hours so that Rosie thinks she has her domain back, Feliway diffuser can help as well. If she weighs enough to get spayed then that can help as well. She is gorgeous 
That's the thing tho Liz, Rosie is barely getting to see her, as soon as she does she starts hissing and gurring

that is when we take Ruby out of the equation, Ruby comes across as inquisitive and wants to get closer, Ruby spends 95% of her time in the living room atm, at night she is put in with the bairn (litter tray n food too) so that leaves the rest of the house for Rosie, but as far as I can see, she is sleeping in the kitchen either on the table or the chair (I've set up a trail cam) We did have a Feliway diffuser years ago, think I gave it to the cat protection, anyway, Rosie doesn't seem to be spending enough time inside for it to take effect

I've currently got a GPS tracker fitted to her, she is currently in the Garden