Change the spawn point for LAN players in a single player map
Pretty sure you can't do this without using a world editing program. Just have your wife join, teleport her, and have her sleep in a bed.
You can teleport like this:/tp [wife's username] [your username]
Edit the spawn coordinates in the level.dat file for your World. You can use MCEdit to do this very easily.
As of Minecraft 1.7.2 you can use the /setworldspawn command to do this without using external programs.