When you enter a new sibling or a new contact for an existing sibling, you will be asked if you want to connect the existing contacts.

So, if say, 2 siblings have different dentists, when you add the dentist for one sibling, you can refuse to connect that dentist to the other sibling. Meaning, add the dentists individually to each sibling.

If you want to remove a contact, hover on the name of the contact and click on the trash icon. This will disconnect the contact from only the selected child.