How could the incident (your child escaping the garden and possibly being in danger) be your fault? I am guessing you mean your son was upset and that was felt to be your fault?
In your shoes I would divide and conquer.
Speak to your young son and explain how dangerous it could be to be out by the street. In your shoes I would put a #consequence* in place. Maybe removal of a special toy (not a beloved teddy!) maybe racing car or train track for a day or week, something he will miss!
Speak to any teenage children separately to explain it is not their place at all to make you feel small (as this is how it sounds) for worrying about their little brother.
Could I ask, you can ignore, do the family normally assume you worry too much? Is this a second marriage (e.g. is he their brother or half brother, this should not make any difference at all but maybe they are less attached to him - I don;t think this is the case, I just think they are less aware of dangers as teenagers!
Speak to DH when kids are in bed, explain it is both worrying he let this happen and worrying he has not backed you up in your concerns.
Maybe he feels guilty?
Maybe he is he just not aware of dangers?
If necessary find a few stories on line....
This is very old, 2002, but it does chill me somewhat "A three-year-old girl who wandered out of a play school yesterday drowned minutes later after falling into a nearby pond."
www.telegraph.co.uk/news/uknews/1414650/Girl-of-three-drowns-in-pond-near-nursery.html
Just as an aside I would also get the gate set up so it really is impenetrable and check out plants etc so your garden is mega safe.
www.telegraph.co.uk/gardening/gardeningadvice/7974916/Garden-safety-pretty-but-poisonous-plants.html
But you are nto wrong to be fearful of a young child getting out of their garden and do not allow your family to make you think this is your fault.
Hey, lots of questions in my post, ignore them, they are more to think through the 'issues' and I do not need answers for me. I am very sorry you have had this happen and hope all will be calm now (and the newspaper article will not upset you, that is not my intention!).