Ideas:

Dear Spike.

How have you been? How have the girls been? And what about your daughter? I think that the last time we saw each other was the day of your daughters birth. Congratulations by the way. And that was a whole five years ago. Far too long I must say. And with that being said I would like to invite you and the girls to visit the Crystal Empire for a weekend. Think of it as a vacation!  Me and Shinning can't wait for your response.

With love, Princess Cadence.

P.S. Make sure to bring little Stellar Fang along as well. I miss my niece.

Dear, Princess Cadence.

It was great to get a letter from you. The girls were really excited to hear from you as well. Especially Twilight. We've talked it over and we have decided to take you up on that vacation. It will be like a second honeymoon. We will be arriving in the Crystal Empire in two days.

Likewise, your friend, Spike

P.S. Stellar just can't wait to see you.