The male voice says oppa? The very essence of the words 'hyeong' and 'obba' is that hyeong is used by male speakers and obba by female ones. You're teaching people how to not speak Korean correctly. You should use your brain more when creating exercises.
OPPA doesn't mean brother! It is USED with: brothers, friends, cousins and boyfriends!
Duolingo, it is wrong to translate it as brother. It is a super weird translation to call your boyfriend as brother