Oh god don't google - you will be terrified of what you might find
Sorry to be thick - but is it your cat that has the twitching problems? if so is this Daisy - I should really have looked into your posts before posting...
ETA right scrap that its your other cat with the twitching. Only reason I asked is that nibbling fur could be a symptom of the issues she has. But as its not the right cat.....
Right ....here's my thought, anal glands. She has been having runny poop has she not? Problem can arise with anal glands filling and not naturally emptying because of no solid poop pressing on them enough to do so. In turn they can get irritated/infected and the animal will nibble at the back legs.
Have a look at her bum and see if around the opening to her anus there is any swelling redness etc - keep an eye on the nibbling, but if it continues/gets worse etc...pop her to the vet for a check over.