Q.P.
I agree totally with your response to your older son. He should have been more responsible as a older brother to look out for his younger brother. We as parents (I believe) have to teach their siblings to be "your brother's keeper".We can not be or see everything and it's always a good to have another set of eyes and ears with the people you care about the most(our kids). Personally I think that the mother that choose to put judgement on your parenting skills was totally out of line. Your son will not hate you for teaching him how to be a good older brother; although he will love and cherish the mother that taught him how to be a responsible young man.