Another child can only be added if the sibling goes to the same school as the child you are already linked with.
To do this, first go to Settings in the Sdui app via the drop-down menu in the top right corner and then open your profile. There you will find the item “Linked Children”.
There click on “Add child”.
If you click on this button, a window opens in which you have to enter the activation code that is on the account slip that was given to your child.
The link is established when you enter and confirm the code.