Kristen and Justin, thanks for the quick replies! Kristen, Marketo can see the contacts (like mother, sibling, etc.), but it's not currently able to see the relationship type or that there is a relationship at all. Justin, I'm trying to make sure to email the student/applicant record for most marketing emails, but certain emails should be directed to the parents. Right now, all of these records parent, student, sibling of applicant are coming down to marketo with little to distinguish who is what. I'm working with the Salesforce admin to get a field on contact. BUT! I will first try enabling the sync for the relationship custom object and see what new triggers and filters show up. If that works, that'd be amazing!
Thanks for the thoughts; I'll let you know how it goes.