Poor girl, I hope she continues to improve and that she is gaining weight is a good sign. I wouldn't worry about her not making noise when she "meows" - some cats never do. We once had a cat who adopted us when he was about 6 months old - much older than Phoenix, and he'd just open his mouth and nothing came out.

I would schedule an appointment with the vet for Phoenix, just to have her checked out. I hope your sister does get her cat spayed, it'll be better for her, and everyone around. It could be she just wasn't healthy enough to have kittens again so soon, or, as she's had no vaccinations, she had some illness she passed on to them, sadly.

By the way - we worried "Cat" would never learn to make noise, as his best buddy became out Saint Bernard, and cats are not equipped to bark! When he finally managed to make a noise, it was an exact mimic of the squeak the cellar door made!

I wouldn't worry about her development because she's an "only kitten," just make sure she gets and stays healthy, and give her plenty of playtime, and she should be fine!