Writing a letter to the bride's parents is a kind gesture that shows your appreciation for the love and support she gives your son. It's also a good way to tell the bride's family about yourself so that they'll learn more about you and your son. When writing the letter, write in a respectful tone even if you don't like the bride's parents because this will leave a positive impression. Also, avoid any language that's sexist, culturally offensive or religiously insensitive.
Express your joy about the upcoming marriage. Mention that based on observations of their daughter's relationship with your son and in conversations with her, you know that she loves your son dearly and that you're happy to welcome her into your family. Also include specific situations regarding this. For example, discuss the time when your son struggled with cancer and their daughter visited him daily and prayed with you and your spouse for his recovery.
Include some interesting details about your life without being too revealing. If the bride occasionally volunteers on the weekends by preparing and serving meals to homeless shelters' residents, talk about how for 20 years you operated a tutoring service for neighbourhood children from your home, and you admire the bride's passion for serving others.
Offer to meet with the bride's parents. In the letter, mention that you want to set aside an afternoon before the wedding for both families to get together and have rehearsal dinner or a picnic in the park for both families to attend.