Ustadhi حفظك الله, I have a Young sibling raised unIslamically in kaffir country. Recently found out how corrupted they were & punished them as parents ordered. The parents acknowledge they are blameworthy. The sibling barely prays & is ignorant of Arabic, anyway to fix a terrible upbringing?

Yes, it needs to be with wisdom not instantly punish, etc. It should be through teaching and setting a good example. Take them to the masjid with u be kind with them, if they r young play with them but your presence there is good thay no haram is to be done may Allah protect us and you ameen.