Union Table with a flag?
Well you would need a family table to begin with: 

create table family
(
  family_id integer primary key, 
  name text not null,
  .... other attributes of a family ...
);


And a table that links a patient to a family

create table family_member
(
  family_id integer not null references family,
  patient_id integer not null references patient
  ... maybe even a date range when this patient was part of the family ...
);


Then you can link the diagnosis to the family the same way you linked it to the patient:

create table patient_diagnosis
(
  diagnosis_id integer not null references diagnosis,
  family_id integer not null references family
);