Model a family tree database in Access 2007
Start using a ERD tool, or use paper to draw the relationships, those you have already written. 

For example, "A tribe has many clans"  and "one clan, which belongs to one tribe" means there's a 1:N relationship between Tribe and Clan, so there should be a tribe_id in Clan table (FK to Tribe).

Another issue would be the "a family has many Sims" and "a Sim belongs to at least 2 families". You'll have to use a Sim_Family joint table for the many-to-many association between the Sim and Family but the "at least 2" is hard (or impossible?) to enforce with DRI alone.

After you have drawn all relationships between tables (entities), check if there are circular paths in the Foreign Keys. It's hard to deal with that and there's usually a way to avoid it by minor alterations in the design.