If your significant other suffers a death in the immediate family, you are forced into a difficult situation.  When it comes to attending the actual funeral, just remember, you are not a member of the family.  And to assume the status of a family member without the sanction of the family matriarch/patriarch is social suicide.

Better to sit in the back of the service and wait to be asked to move to the family section than to sit in the family section and be asked to move to the back.