Scheduled reminders triggered by new relationship
I think using the CiviRules extension (to be found at https://civicrm.org/extensions/civirules) will help you with what you want to achieve. 

You will also need to install the Email API extension (found at https://civicrm.org/extensions/e-mail-api) to be able to use the CiviRules action to send an email. 

I do not know if the trigger 'new relationship' has been thoroughly tested, but you start should be to read the CiviRules documentation and see if this might do the trick.