Oh I am so sorry to hear, because everything seemed to turn out so very well with Luna and Lily...
I also think that it is a medical problem and I do hope your vet will help!!!

I've read that you can place aluminium foil over the couch, that will keep the cats away and you can give them access to the living room again...and maybe placing one of the litterboxes close to the couch may be helpful as well...
I hope that it isn't a behavioural problem with Luna, because it is would be so hard to deal with it for you right now...
Whatever it is I hope Luna will recover quickly and you'll enjoy your little "Katzenglueck" again!!!

Thinking of you, Luna and Baby Lily,

Sabine