I'd take it carefully with the brother in law. Have you considered the possibility that he might think you deceived his sister when you married her and have been gay all along? I'm sure it's more likely that he will know your love with his sister was true and will be a good sounding board for you.
It could be an entirely different problem for you if he's always had the hots for you which he now wants to explore, but you don't share those feelings.
Just trying to say it could be a tricky conversation that could go in a direction that you don't expect.
I'm a big fan of your posts and your family so I hope I'm totally on the wrong track.