When my female cat used to go the vets, my male cat, no longer with us, took a few weeks to get close to her again. He would just hiss at her when she came near and she loved cuddling up with him.

If they are related, it isn’t unusual for some sibling cats to grow apart as they mature. It’s sad for your cat C as she doesn’t like other cats so is rather lonely at present.

I would suggest using some pheromone sprays or plug-ins around the house which may help.

Hope with more time, things will go back to how they were.

 

One of mine always sniffs suspiciously at any of the others which just came back from a vet visit. The dog I had used to do it too. I think some of it is that having been handled at the vets they smell slightly suspect.

 

Very common problem. Lots of scent swapping - put a glove or a sock over your hand and stroke each cat in turn, especially around the cheeks.
Feliway Friends is probably the best choice for a pheromone plug in.