I'm a bit sad about it . Adopted 2 female cats over a year and a half ago when they were 6 months old. They were not sisters but together since small kittens. Rescue said that dcat1 was the only other cat that dcat2 would tolerate (I guess this is very relevant now!). They were close up until a couple of months ago, at a guess. Would sleep together (not all the time but quite regularly), groom one another, dcat2 would follow dcat1 around outside. But dcat1 has now taken to hissing at and batting dcat2 if she comes anywhere near her or if she wants her to move off my knee or get out the way. Actually it's a very weird noise she's makes, almost like a series of small sneezes that turn into a hiss at the end 
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Dcat1 is a tricky customer, very nervy and won't let the kids touch her. She likes me and formerly dcat2. That was it. There were signs of them becoming less close gradually to be honest, again on dcat1's side . Less and less time together and not mutual grooming as much but it's such a shame she's now being openly hostile towards dcat2. I think I know the answer in that there's not much to be done but wondered if anyone had any advice? Thanks.