It's not just restricted to cousins and families.
I used to volunteer at a Islamic supplementory school and everyone there would refer to each other as 'brother' or 'sister' ( I know its different as it was obviously out of respect)
I would always say, 'I'm sorry but my name is not sister, its Milly.'
Anyway, one Saturday, Sister Jabeen (not real name) told me she found brother Jamal (not real name) attractive and was wondering how she would go about telling him.
I just found it slightly strange. I dont call anyone but my brother, brother. Even then I dont, hes younger, I refer to him as 'oi'
MOG, that sounds just like my university!!! How we used to laugh at those very thinly veiled brother / sister relationships!