In Law Greeting Cards What to Write

Just stay personal with your cards, write out how you feel, and go back to a few good memories!

I agree with Brittany! Make it personal! Relate back to some memories that mean a lot to you that made you feel a part of the family and how excited you are! Promises to them about their son for sure!
One thing I want to put in mine I've seen online is "thank you for raising the man of my dreams" it won't be exactly that but along those lines I always thought it super cute!

If you're making them yourself... just try writing something personal for each person. Talk about what they mean to you, what you're thankful for, and maybe your promises to their son.
You could even include any personal jokes or light humor that you have shared with them.
The important part is to show them that you are officially family, and showing your love for them.