Maybe JI is not furious because he knows that DB is the good mother that all of the people who know her seem to think that she is. Maybe he believes her because he knows in his heart that she could not/would not have harmed the baby. Maybe he realizes that life is full of coincidences and that when you attempt to reconstruct incidents after the fact, a lot of things that would mean nothing on a normal night, can be twisted to seem sinister.
That's not to say that he would necessarily be right, but it would explain why he supports her. Actually, I think it's very interesting that no one in his or her family, except that one uncle or whatever, even hints that they don't believe her. That's rare, in my opinion. Usually in a case like this, with this much negative public opinion against a parent, there would be people from the other parent's side starting to show some doubt. If the people closest to them are not wavering, that would seem like maybe Deb has some rather positive mothering skills, and is not just the box-wine-drinking, leave-the-baby-in-the-crib, neglectful, irresponsible mother that some people interpret her to be.
JMO