As above, you need to tell your MDE account that the person of that name in your F&F list is the same person who has an MDE account under that name.
Go to your daughter’s name in your MDE. Tap on her name and scroll down to Connect with This Guest, then tap Send Email and fill in the email address she used to create her MDE account. She will need to accept that, then her tickets and MBs and any other current plans you have will magically move to her MDE account.
Then go to your son in law’s name in your F&F list and tap on it. If he has his own MDE account the process will be the same. If your daughter will be managing his profile then you’ll be sending the email to her MDE account again and she’ll have to accept for him.
In either case, make sure that all of you give permission to plan for each other, to share plans, and to view each other’s photos.