I think you need add a join to your query, something like this:
.join(LargeGroupAttendance.attendee)
so that the final query would look like this:
attendance_records = (db.session.query(LargeGroupAttendance).
filter_by(large_group_id = event_id).
join(Attendee, LargeGroupAttendance.attendee).
order_by(desc(Attendee.first_name))
)
See SQLAlchemy: How to order query results (order_by) on a relationship's field? for a more detailed explanation