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'
its bit grey area, but i have seen people fancy each other while calling sister and brother in such islamic scenarios. what i notice some start calling /addressing each other like in full ie "Sister in Islam" and "Brother in Islam" making sure they are not calling them just sister and brother.... :)
for cousins, I say it depends what kind of feeling one have towards specific cousins. while calling me "bhai" two of my cousins wanted to get married to me, and one of them had a short affair with me.