I think that's very common in South Asian languages. Same goes for uncles and aunts. You just extend the family outwards and carry relations with as much fidelity as you can. So a lot of "brothers/sisters/uncles/aunts" are children of my aunt's (through marriage) brother, for example.

Edit: I bet there is a term/prefix for "direct" relationships in Tamil. In Telugu, for example, everyone is "anna" or "mama" but you can use prefixes like "mena-" i.e. "mena-mama" to imply it's a direct uncle. But you only use that distinction when you directly need to specify the relationship. You'd still call them or refer to them as "mama"