Sister cats both licking the hair off of their stomachs and tails

Hello,

I have three cats. Two sister cats - Ruby and Agnes - and a younger cat (as well as a dog...)   Well, the two sister cats - both will be 6 in January- seem to be obsessed with overgrooming.

It started with my cat Ruby. About two years ago, I noticed she had licked almost all of the hair off of her stomach nearly overnight. I took her to the vet, there are no signs of fleas, her skin seems fine, and the vet passively diagnosed her with feline hyperesthesia and/or OCD, and didn't seem very worried about it. Since then, Ruby has maintained her hairless look on her stomach, but nothing has escalated from there. 

A couple of weeks ago, Ruby's sister Agnes has started licking the hair off of her stomach! Now I'm worried something genetic is going on? She has no signs of fleas or mites, but has licked the hair off of her stomach and around the base of her tail. Reading more into feline hyperesthesia, one of the behaviors is a "shaky tail" which she tends to do when she's excited or when I talk to her, but I never thought it was a bad thing.

I talked to my vet again, and she suggested switching to a limited ingredient diet. My animals have always been on a gluten free diet, but now I've found a gluten-free LID limited ingredient diet by Instinct, which is also grain free. http://www.naturesvariety.com/news/46    I'm hoping for some other suggestions, because I don't want this issue to get out of control and I want my babies to be comfortable and happy.

My other animals show no signs of skin irritation, and I see no signs of fleas or mites, so I'm thinking it's something else. All of my cats are indoor only, fed in glass bowls, filtered water, grain free food, given lots of love and attention, with lots of room in my house, multiple cat trees, toys, etc etc.  

Any suggestions or ideas would be greatly appreciated. Thank you!!! 

It could well be an allergy.  Along with the limited ingredient you also want it to be a novel protein.  If they normally eat chicken then get one in another protein like lamb or rabbit.  If they have developed an allergy to the protein they are eating now it still may take a while before you will know that the new food is taking care of the problem.

Thanks for your response. Before I was feeding them grain-free dry food that tended to predominantly have chicken, salmon, or tuna. I was also feeding them grain free wet foods that were mostly seafood based. I have switched to a grain-free limited ingredient dry that has duck, and have removed all seafood from their diet since I read that could be a common allergy. They've been on this new LID diet for about a week now so I'll see how it goes. Thanks again!