It's funny. A Tamil friend of mine from a specific group would not differentiate between her 1st cousins and her siblings. So her female cousin was her "sister" and her biological sister was also "sister". Likewise for brother.
She was surprised to learn that we don't recognize the similarity of those relationships.
I suspect the appearing naturalness of this is all down to whether or not you grew up with it.
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"
She was surprised to learn that we don't recognize the similarity of those relationships.
I suspect the appearing naturalness of this is all down to whether or not you grew up with it.