How do you prove it? Marriage is that contract. If someone is willing to marry, why are they willing to sign a different contract that marries them? How is that other Contract different and better?
There are times with Medical Conditions you must prove concretely you have the rights you need to demand. Blood Family can override any Contract, but, actual marriage.
I honestly, genuinely don't know what you're talking about here.
In times of medical emergencies, the patient's medical record should specify (through setting up ahead of time with their insurance company or whatever) who they want to visit them in the hospital. That may include blood, that may include their spouse, or that may not. If a person specifies that when they're in the hospital, he or she doesn't want family (who the person hasn't talked to in 10 years) or spouse (who the person has been separated from for 3 years) to visit him or her but his or her closest friends or whoever, they should absolutely have that right. Blood and marriage should not get priority as a "one-size-fits-all" solution; who-gets-to-visit-you-in-the-hospital should be taken on an individual basis based on what the one-day-will-be-sick person wants.
I really have no idea what you're talking about in terms of contracts and the like.