To turn off a guardian/contact for one sibling you would go to the Family tab > Family List > Click on the child’s profile > Click the drop down on the guardian/contact profile > Select Manage Guardians or Manage Contacts. From here, deselect the guardian/contact that you wish to turn off for this sibling and click Done when you are finished.

NOTE: 'Turned off' guardians and contacts will still be visible in any guardian's parent portal account.