My brother's wife is called my 'sister-in-law'.
This term has a dual meaning in English, as I can call my brother's wife my sister-in-law, and I can call my wife's sister as my sister-in-law too.
Then my question is to you, if I want to tell someone about my brother's wife that,
"My sister-in-law is a business women"
How they could understand, is it my brother's wife or my wife's sister?
I agree with Seth. It's not an important distinction for English speakers. If it were important, we would have different words for the two concepts. That's how language works (generally).
If someone really is interested to know whether you're referring to your brother's wife or your wife's sister they can ask you.
It shouldn't matter. If you say your sister-in-law is a business woman, few people would ask if she's your wife's sister or your brother's wife unless they know you or are interested.
It's the same in quite a few languages.
I agree with Michael and José.
@Seth : Thanks for your question. Yes, In Bengali language we have different words for dad's brother/mother's brother.
I can give you some example.
In Bengali language Mother's brother is called as "Mama", Mother's sister is called as "Khala", Dad's brother is called as "Kaka", Dad's sister is called as "fufu" etc