Here's an update:

After a horrible night for everyone of us (Luna feeling lonely in her "prison", Lily missing Luna and mommy being upset anyway), Luna finally used her empty litterbox this morning! Now I could finally open the door to the cat room again and Lily and Luna were greeting each other as if they have been seperated for ages!

However, I immediately brought the urine to the vet, and they found out there's a small amount of bacteria and an increased pH-value - so it very much looks like a beginning UTI again. There are no struvite crystals yet, but due to the basic pH it's very likely she would have devoloped some in the next future. I've got pills for her now and have to find a diet that allows the additional feeding of Hill's Indoors (which she has to eat, otherwise she constipates again). Well, things are always complicated with my Luna girl!

At the moment, Luna and Lily are happy to be together again. I just found Luna grooming Lily and the baby was suckling again! They are definitely happy to have each other!

Kirsten