Thanks for all the bounties.
BridgeWalker, we've bought a house, so there's only one office.
I mentioned it to my wife briefly tonight, and she said that both BIL and her sister are against the idea. I don't think my wife has pushed it very much, though, and I don't think that BIL has thought it through (or is aware of just how much can be done).
I could push it now, but I suspect it would just make them stand their ground more firmly. I think a better tactic would be to give sister a few weeks to really screw things up (the way she had them royally screwed up when they hired my wife), and then suggest it again. By that point, BIL should be more receptive.
Sister does things like forgetting to pay taxes, pay employees, file union papers, file tax returns...all that stuff that keeps a business going.