Membership status is the institutional fact of taking someone to be part of some group by some collectivitiy for some time. 

There can be a clash here with traditional CRM modelling which has decided that couples are groups. So when you model the relationships man and wife do you model it as a membership status or as a family status? I would argue for family status. The group function always seemed like an overly romantic interpretation for ontological modelling. That being said both could be justified.

The deeper ontological question. Is family status a subclass of membership? I would argue no that they are not overlapping phenomenon. You purposefully join a group, you simply come into family relations.