How do I put siblings in the same family?

1. Open the second child's profile.
2. In their family block, tap Change family.
3. Choose the first child's family in the list, then Add to this family.

That family's parents then apply to both children, with a single email. If the old family has no members left, its parents follow the child into the shared family, at the end of the list; a parent already there (same email or same student profile) is not added twice, the duplicate goes to the trash. The old family disappears from the lists.