How to invite someone to my iCloud Family Sharing without them losing their data

If your daughter is sharing an account with her husband (which is bad practice and they really should each have their own account*) then that is seen as a single thing. You don't need to invite them both. You are actually inviting the single account.

Transition should be automatic when the daughter/husband account joins your family.

Read this article to learn how to share iCloud+ with your family --> Share iCloud+ with your family - Apple Support

"When you share iCloud+, any family members using the free 5GB plan are added to your family plan automatically."

"If a family member is already paying for their own iCloud+ plan, they can choose to switch to your plan or keep their own and still remain part of the family. When they switch to the shared family plan, they get a refund for the remainder of their personal plan. They can't keep their current plan and use the shared family plan at the same time."

*Make sure that each family member has a unique Apple ID --> Make sure that each family member has a unique Apple Account - Apple Support

EDIT:

If what you mean was your daughter and son in law each have their own accounts that are currently part of their own family group, they would have to leave/disband the group in order to be able to join yours. Read the section "What happens when you downgrade or cancel iCloud+" in this support article Downgrade or cancel your iCloud+ plan - Apple Support Apple's documentation is somewhat unclear as to what would happen to iCloud data during this transition. One reading of their information is that the data would remain in an account but the owner couldn't add anything more to it until the owner was part of a higher level subscription if they were using more than 5GB. In that case there would be nothing to worry about during the brief time when an account was not part of a family group. That's my reading of it. If one wanted to make sure there would be no issue, the account owner would take out a personal higher level subscription before leaving the first family to bridge them over during the transition period. That way there would be continuous coverage until they were part of the new family sharing group.

Read this support article about managing membership of a family sharing group --> Leave Family Sharing - Apple Support