The cat is currently licking the carpet in a corner of the living room, her sister was asleep in it before. Its the one part of the room they will pick at the carpet in and they often chose to roll around or lay there.
BUT LICKING?! Its a concrete floor with carpet underneath, nothing new or special about that area of the room.
I've only seen cats behave like that over catnip or opposite sex cat has scented there, I would try getting some catnip and seeing if they like it (as some cats don't) and if they do you could get a scratching post or bed and put catnip on there. If you haven't already maybe get them spayed/neutered, unless you either want kittens or want them wandering off and then coming home and spraying/marking. Obviously male/female options as appropriate! Finally all cats sharpen their claws they like wooden posts so if you have a garden you could put a scratching post outside, be warned though neighbouring cats may also take up the offer! As to why they like that part of the room...habit I would say so if you are going to encourage them to use an alternative put it there! Good luck
Very strange, they are by no means old but could be a diet thing, maybe a quick visit and health check by the vet, blood test to check liver and kidney function. Some animals behave very strangely when they feel unwell. I thought about this the other day after I left my last message and remember a orphan kitten we had and he was abandoned by his mum when v.v young, he formed an attachment to a hat and carried it around to knead and suckle at, he lived to 22 and always had his hat and always behaved like that with it. Your cat may have a kneading licking thing due to early separation...? You could try a change of diet as a precautionary , cats love real fish, sometimes gives them that boost they need, also could try a little rub of tuna oil on their paws next time you have a tin will keep them busy for hours cleaning (smelly I know) but its a diversionary. All cats love to be clean and sometimes fish oils can keep them amused for hours of cleaning. I really would suggest putting something over this spot in the carpet as if it is smell based it could be your only hope to stop this behaviour. Final thought there are products on the market supposed to deter cats and smells they don't like, worth a chat with the pet shop to see if they have anything. I would move furniture first though! Hope this helps