If your spouse is in a coma who makes their medical decisions?

A friend of mine who is married is INSISTENT that if she were to be in a coma and unable to make her own medical decisions that her brother not her husband would be asked to make all her medical decisions. Doesn't the law state that your spouse is your next of kin? I've given up arguing with her because she's insisting she's right. I think she's wrong. She doesn't have a living will or any kind of advance directive stating her brother would be in charge. So what is the right answer? Is her husband or her brother in charge?