In The Netherlands, much information is retrieved from "leenregisters", showing who was renting a property. Actually having loaned from a lord also implied that you supported him, like when he needed fighting men. The ledgers cover roughly 1200-1500 (I think; I'm not an expert). In these ledgers many references are found to relatives (wife, son, nephew, etc.) and each entry is dated. For example, see http://members.casema.nl/r.vanderkrogt/ovinhoud/index_l.html
https://docs.google.com/viewer?a=v&q=cache:B8zaL3SUaiYJ:www.hogenda.nl/docs/Attachment.aspx?ID%3D%257Bc89bdab6-910c-48ee-84fc-0a473bc6c4b6%257D+leenkamer+van+de+graven+van+blois&hl=nl&gl=nl&pid=bl&srcid=ADGEESizqxgJi7gkOz3_3CVDSbhY7OXslIYAKwVNMfOCG857ZYwVH1kIs7lEWYWJ3pu7vAh2m27P1Ast7hAh_5NOBMFHKC3cmjdHCBtRzz-WXGtRuaB5T6Dd3YjlVXfLUvDnvudiqBiK&sig=AHIEtbS8ZwZcZOyZgWbTfg2h00_hiQpOSw (sorry: google for "leenkamer van de graven van blois" may be faster)
These important sources of information should be a type of event on the Person pages, and we should standardise the reference to sources: both the originals (often hard to read) and transcriptions. I'm convinced similar systems and registers were kept in other countries.