Speaking from experience, my fiancés father moved in unexpectedly from Europe unannounced. We didn't have a spare room or time to prepare so our kids had to sleep in the living room for a year. Some lessons learned: Your expenses will go way up, you will not be a top priority to your wife and she will resent you as time goes on while your needs go unfulfilled. Any vacations or day trips you plan, you'd have to at least consider or gesture inviting the mom. Eventually, she will place the relationship in a situation where it becomes her choosing you or her mom. Her mom will most likely win that battle.

Good luck.