Inserting details of another contact as tokens into an email
Are you using drupal as your CMS? We have some custom tokens with token data from the organizational record that individuals are related to.

Check it out here, it won't get you all the way as we are using custom fields and relationships not applicable but you may be able to use it as a template of sorts:

https://github.com/elisseck/civi4.6-related-info-token