We once had Smokey, an old male and brought home Eddie, a young cat - also a male. This male "nursed" on Smokey for years! Smokey even grew large nipples. It just gradually stopped - guess Eddie finally got the nurturing he had missed. My dad sure liked to make snide comments about this as you can imagine! Our brother/sister kittens are now 10 months and Emily "nurses" a blanket every night but we notice she is starting to do it less and less before she drifts off to sleep. I don't see any problem with it. I think it is sweet. Lily must just need that security still and Luna is a good mommy surrogate.