To refer to your family member who is older than you or a family member with higher position than you, you do not use 나의 [na-ui] or 내 [nae] (my …). You use 우리 [u-ri]. Example: my father = 우리 아버지 [u-ri a-beo-ji], my older sister (the speaker is female) = 우리 언니 [u-ri eon-ni], my older brother (the speaker is male) = 우리 형 [u-ri hyeong].
It’s not really wrong to use 내 언니 [nae eon-ni], but it’s just weird. You will sound unnatural.
To refer to your family member who is younger or your equal, you may use 나의 [na-ui] or 내 [nae] or 우리 [u-ri], but 우리 [u-ri] sounds more intimate. Example: my little brother = 내 동생 [nae dong-saeng], 우리 동생 [u-ri dong-saeng].
To refer to ‘school’ (=학교 [hak-kkyo]) as a place you belong to, you use 우리 [u-ri]. Example: my school = 우리 학교 [u-ri hak-kkyo]. 우리 회사 [u-ri hoe-sa] (my company) means ‘the company where I work’. But if you say 내 회사 [nae hoe-sa] (my company), it means ‘the company that I own’.
But then, why BTOB sings 내 아버지 [nae a-beo-ji] in their song ‘Father’? I don’t know… but I remember I heard that bending grammar rules is allowed in art. *Don’t just believe me, you should ask a Korean to confirm about this. ###