sobieski queried:
But what is the difference between bratowa and swajgierka then ? :(. Dictionary gives for both "sister-in-law"
According to the grandfather of all knowledge (also known as Wikipedia)
"Szwagierka to siostra współmałżonka, bratowa to żona brata. "
Szwagierka is (your) spouse's sister. Bratowa is (your) brother's wife.
It goes on to say that szwagier (from German IINM) is either your sister's husband or spouse's brother.