To Johannes, the reason the Shimazu bros were never kidnapping anyone was probably because you had higher priority events that displaced the defection events. If all goes to, well, quite frankly, shit, then you will just have to keep churning out those turns until they do defect someone - or, even worse, start from an earlier save point. Usually, there is a defection when Suzume warns Rance about the Shimazu bros right before the pre-turn event. So exactly why they haven't been kidnapping, I couldn't tell you for sure. Sadly, it's up to the game whether or not it feels like allowing the kidnapping event to happen.
But beware - once it DOES happen, there is a chance that they will kidnap someone else before you get the event that shows Rance in distress that the Shimazu bros are taking his bitches and Suzume suggesting that they kidnap Kurohime in return.
Keep at it - only in this route and Kill the Monkey can you recruit the Shimazu bros, and they are fucking worth it.
For information on Daiteikoku, I've posted some info on another thread with WhiskyWolf and others in it explaining some basics on the game. And yes, it's gonna be a bitch to translate - one website claims that Daiteikoku is the biggest project Alicesoft has ventured in, but it has voice acting (at least for part of the dialogue). Here's the link for that thread:
http://rpgcodex.net/phpBB/viewtopic.php ... 52#1643352
WhiskyWolf is right, they must be fuckin outta their minds if they don't translate this - Daiteikoku is already an awesome game even if it isn't translated. But playing it is the problem - that's where I come in handy :D