I hope your uncle gets well soon.
According to the things you mentioned, he is probably having what we call Hospital Acquired Pneumonia. If he got it while staying at NHSL, that's bad news (NHSL is where the most serious patients end up). You need to be more vigilant about his condition. If you are not satisfied, it is always better to seek a second opinion.
Anyway, since this doctor you mentioned was on duty inside a ward on a public holiday, I'm guessing she is a House Officer (Training Period). If so, she is not entitled to tell you a diagnosis unless her VP has made one (he/she probably hasn't yet). If you have an issue with the diagnosis, you should talk to the next level (Registrars and above).
Her behaviour is highly unacceptable. If you're going to make a complaint, better wait till the patient is discharged.
Hope this helps.