There's quite the close relationship between BIL and LW, apparently, since he's been telling LW all about his prostitution adventures for a year?

If I were LW (but with my values), I would tell him "I don't have a problem with the prostitution, but my sister deserves to know. Tell her or I will, you have one week."

And then I'd grieve the loss of any good parts of my relationship with them, because whatever happens that relationship is gonna change, even though it's not my fault.