So, let me make sure I'm hearing this correctly. You have no problem with changing the sleeping arrangements, but you do have a problem with WHY she wants to change the sleeping arrangements, and do not believe that the wife is being forthcoming about the real reasons that she wants to change them?
How about if you distill the post you just made into a few key points (maybe use an index card) and call a "family meeting" (sans children) to discuss it?
Other people here will have all kinds of other advice, but at the end of the day, that is what you'll have to do: talk about it with the people involved. There is only so much you'll be able to accomplish on an internet forum.