//Also occasionally: the husband of one's sister-in-law."//
The thing is, Corby (as with the "second cousin" issue) what is it these dictionaries are trying to do? I understood their function was to provide the definition of words. But it seems they have stretched this to the misinterpretation or wrong use employed by some people ("occasionally" in this case, "more loosely" in the 2nd cousin example).
There is a clear definition of brother/sister-in-law and it does not stretch to the spouses of those relations. I know that language evolves but there must be a distinction between evolution and plain incorrect usage. It seems to me that these publications are complicit in encouraging the latter by legitimising wrong usage under the guise of "loosely" or "occasionally".