Having worked EMS in two states and trained in multiple hospitals I can speak about two aspects. If a patient is critically ill, the care they require often necessitates the exclusion of a family member, etc. This is either becuase there just is too much to be done too quickly and not enough space to do it in or things need to be done to and for the patient that the family, etc doesn't need to see. Further, believe it or not, they often lie to loved ones and sometimes the only way to get the truth out is to seperate them from family, etc.
Just as much, while the rules may state one thing, how they are applied is completely different. Play nice and you'll usually get your way, take an attitude and the rules about who can and can't be with a patient will be quickly enforced.
If all else fails, I've told more then one gay couple "if worst comes to worst just tell them he's your brother", etc. I've just as easily left blood family members at the scene without telling them where we're going (we're not legally obligated to tell anyone). Again, its all in how you play the game.
