how would "an American sister is weird" be different?
أُخْت أَمْريكِيّة هي غَريبة
It depends on the context of this sentence and what you're trying to convey. If you're referring to a group of American sisters and non-particular one of them is wired, then you would use the same Arabic sentence used above.
Y can't it be "An American weird sister"
Is " a weird American sister" wrong here?
What do you mean? It's the answer.
No, it is correct.
why does its pronunciation be different from what was taught in the beginning?
A weird american sister
I did put a weird american sister. How did i get it wrong?