so you are doing your best for them as I can tell by your post ..so I would say its jealousy but get any pain checked out .have you any pics up of them ?
I live in the country in Mn but we do have a few neighbors close to use. One of them really hates cats and has told me she will shoot any of my cats that might cross her property line. The idea terrifies me. I do have a nice rabbit hutch that my girl like to go sit in when the weather is warm. They like it so much in fact that sometimes I have really hard time getting the out. One of the is leash trained and gets really excited to go out. She just looses her mind when she sees the leash. She runs to the door and meows excitedly Shr almost acts like a dog sometimes. But right now it's so cold out that I don't want them outside. Maybe it's time to switch up some toys. As for the old one I plan to call the vet tomorrow
Frustration maybe, do they get out at all, sounds like jealousy aswell ,if she is meowing a lot as you say she could have some pain ,maybe have her checked out .good Luck