They're related, it was rihtwis in Old English according to EtymOnline. :)
rättvis is fair, righteous. the Finnish word for righteous, ''vanhurskas', comes from 'vaa'an hurskas (=viisas)' and that is loaned from Swedish 'rättvis'.
Fair is more common but also righteous might be accepted even the meaning and use is a little bit different. I knew what rättvis ment and I could have translated it 'reilu, oikeudenmukainen' in Finnish. But I could not remember the word 'fair'. I.e. Duolingo is teaching and rehearsing English along with the Swedish. But that won't harm me, will it?! :D
This sounds like ret-ah-vees, but it is not spelled that way. Is it just a TTS mistake perhaps?