Thank you all.

The initial transition was certainly bumpy for few months for all cats and people, but that was last July and they all seem to get along just fine now. My youngest and newest addition has been to the vet recently, she is in perfect health and I honestly don't think she's the culprit. My 2 older males are due for their check ups. Guess that's the next step to rule out any medical issues.

What exactly is Feliway? I'm always skeptical to gimmicks, does it really work?