"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! :)
"Bibi mereka cantik" should be accepted.
Bibi = Tante
tante mereka Indah was not excepted