I would suggest a call to vet also, at the minimum, because bats are known for carrying nasty diseases - one of which is rabies....ick!

If Lily is not responding to the diet food, the other thing that might help is more exercise. Get her to play more. Someone here once mentioned that when they gave their cats treats, they put them on the edge of an end table so the cat had to reach up on the table and paw at the treats to get them - thus giving the cat exercise.

My only other suggestion would be to always pick up the bowls of food when they are done. If the other two are still hungry, you can feed them at alternate times, without Lily knowing about it - if possible. And, when you do feed them, maybe give Lily just a little bit less than the others. Just like people, reduced intake and more exercise should do the trick!