Halachipedia has a footnote on this, which does not have a citation to a primary source, but according to the footnote, relative extends by halacha as far as second cousin, and by added stringency as far as third cousin. And it includes the spouses of blood relatives. But it does not appear to include the relatives of spouses of blood relatives, so I think I would conclude based on this that the sister of your brother in law would be a kosher witness.
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timeasmymeasure
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Date: 2018-12-27 01:16 pm (UTC)https://halachipedia.com/index.php?title=Kiddushin#cite_note-38