"Tante mereka indah" should also be accepted.
Hai teman! I think yes. Bagus sekali. Since the use of "cantik" is for humans, and "indah" is more used for objects, but can also be used for humans and other living beings.
Selamat belajar! :)
indah just can't be used to specifically refer to human's facial appearance, but you can use it for humans to express for example how someone dresses up, or how beautiful is their hair or their body shape (mostly for female).
"Bibi mereka cantik" should be accepted.
Bibi = Tante
tante mereka Indah was not excepted
indah is more like for a painting or a scene cantik is more towards people