Simply replace the unfilled areas such as "" and "[date]" with your information.
Dear [Name] and [Name],
What delightful news it was to hear of your engagement! This is one of those truly wonderful and rare events in life that you will remember forever.
I can't tell you how happy I am that you have found each other. Finding and committing to one another is such a special honor. As your relationship has developed, I have seen how your affection has blossomed into a powerful love. You compliment each other so well. It is very clear you have a magnificent future ahead of you. I look forward to your wedding day, when we can all share in your joy. Let me know if I can help with the planning.