I recall a long-abandoned fic on a website that I'm pretty sure is now defunct that had a plot element half-way through the story in which Ranma, Ryouga, and eventually Pantyhose Tarou find out that they are half-brothers. Their father is a traveling salesman with Ryouga's curse and the ability to charm his way into lonely housewives beds. He has a "girl in every port" that are usually housewives.
I think he might have sired some other minor characters such as Hiroshi and Daisuke, but I'm less sure of that.
I can't recall what the story was called or what the main plot was, but I do recall that some of the characters were actually starting to work together and settle their differences. I'm also almost positive it was written by a husband and wife team or something like that that shared a web page. I might be wrong, but it's like it's on the tip of my memory but I just can't recall it.
I'm pretty sure I saved a copy, but without more to go on I can't figure out what to search for. Does any of this ring a bell? I know it wasn't on Fanfiction.net and also that the story MORE than four years old, probably closer to 10.