The card simply has a line to fill in our name and then "Number of persons____"
What would you tell her, readers? Click here to read my response.
This is the kind of delicate social situation that I believe requires more than a one-sentence reply. My advice is to write a letter thanking them for the invitation, and stating that due to circumstances out of your control, you will be unable to attend the reception, much to your sorrow. Then say "If it is alright with you, we'd love to attend the wedding ceremony." Enclose the reply card with your letter, and write on the card "Number of persons _ 0_ attending the reception. Number of persons _2_ attending the ceremony.
Note that if the situation were reversed, and you could only attend the reception but not the ceremony, this would be a trickier letter to write. In that case, it could look like you were only coming for the free food, and missing the most important event of the day.