Translation:My brother's wife is a foreigner.
My sister-in-law is a foreigner seems a correct translation if they are married. The correct version I saw was "My brother's woman is a foreigner" which is not very common in the US and has class implications.
The translation of "mulher" in this case is "wife," not "woman," and there is a separate word for "sister-in-law": "cunhada."
It's very difficult to find previous discussion threads in DL, but there was one where I asked in detail about the translation of mulher as wife in English since calling your wife "woman" is US English is pretty crass. Paulenrique explained in detail that it is actually more common in BP to use mulher for wife than esposa, and that there are no "class implications" for doing do in BP. Use of mulher for wife is likely to not sound appropriate to any native English speaker from the US, but I have now take to referring to my Brazilian wife as mulher when speaking to Brazilians, and no one has said a word.
Is it wrong to include "uma"? A mulher do meu irmão é UMA estrangeira."
I don't know if is grammatically correct, but it can be used. I'm not sure if I can express this in English, but it would sound like "my brother's wife is a kind of foreigner" (I'm trying to give a slightly racist accent to it...)