We have a membership type where the relationship type is specified as "Spouse of", with the idea that this would be how we implement a Family Membership. However it seems that only the contact who is designated as the "Primary" member can renew correctly. When the non-primary member renews, it works but a new membership is created rather than updating the old one. This is problematic for us because many of our Family Members are equally active; it won't be the same person every year who does the renewals.
I think the only way you could do this currently is by having the 'family' as an Organisation, use the 'on behalf of' approach on the Contribution page, and ensure all Family members are 'current employee' and have a 'permissioned relationship'.