In Middle Korean texts, 오누이 (with Middle Korean tone marks and spelling: 오〮누의〮) was equivalent to 'siblings' (now supplanted by the hanja-eo 남매).

This is equivalent to unattested *올 'brother' + a well-attested lexeme '餒必' in early Middle Korean and in later Middle Korean as 누의 'sister'.

The former survives in reflexes like 오라비 (< *올 + 아비 'father/man', Middle Korean 오〮라비〮) and, with the vocative -아 suffix, the modern 오빠.

The latter survives as 누이, in Gyeongsang-do as 누부 and in Yukjin as 누비/누배; in a similar process with the vocative -아 suffix and possibly the honorific 님, as 누나.

Did *올 and 누의 ever refer to younger siblings? We do have 아ᅀᆞ in Middle Korean, now 아우 in the standard, for a man's younger brother. But what about a man's younger sister, or any of a woman's younger siblings? It seems the *올 and 누의 reflexs were used in such cases. See here for a previous answer.

In Middle Korean texts, 오누이 (with Middle Korean tone marks and spelling: 오〮누의〮) was equivalent to 'siblings' (now supplanted by the hanja-eo 남매).

This is equivalent to unattested *올 'brother' + a well-attested lexeme '餒必' in early Middle Korean and in later Middle Korean as 누의 'sister'.

The former survives in reflexes like 오라비 (< *올 + 아비 'father/man', Middle Korean 오〮라비〮) and, with the vocative -아 suffix, the modern 오빠.

The latter survives as 누이, in Gyeongsang-do as 누부 and in Yukjin as 누비/누배; in a similar process with the vocative -아 suffix and possibly the honorific 님, as 누나.

Did *올 and 누의 ever refer to younger siblings? We do have 아ᅀᆞ in Middle Korean, now 아우 in the standard, for a man's younger brother. But what about a man's younger sister, or any of a woman's younger siblings? It seems the *올 and 누의 reflexes were used in such cases. See here for a previous answer.