Ponder carefully the transfer request before reacting. Take time with your family to list the pros and cons and decide after that what's best for you.
Share the concerns you may have about the job transfer with your boss. Provide reasons, not excuses, why the job transfer may not be in your best interest. For example, it may hurt you financially and your spouse may lose her employment. Or, this particular assignment may not be part of your career objective.
Explain to your employer that you can contribute more where you are at currently. Be prepared to expound with a business case and show that an uncertain return on investment from the transfer may not be good for the company's bottom line.
Present your boss with an alternative. Perhaps you know of someone else who would benefit from the transfer and actually wants it. Your employer will then have a willing and enthusiastic individual to fill the transfer and you can continue to add value where you are at. Or, if possible, you may propose that you can assume the responsibilities without having to move.
Strive to maintain a positive relationship with your employer. You do not want to come off as someone who shirks responsibility or is not supportive of company requirements.
Work extra hard to show that you are still a high performing employee and a valuable company asset.
Be prepared to lose your job. In other words, have a fallback plan in terms of employment if the transfer is unavoidable and you are certain it will not be beneficial for you and your family. Have your resume ready and interviews lined up so you can give your boss the ultimatum if nothing else works. If you are truly a valuable employee, your employer will try to work something out.