...where was the boys' extended family, school administrators, and child protection and / or mental health and / or judicial systems designed to protect the poor boy.
IMO, government services are set in motion in response to complaints made by individuals, at the right time, to the correct authority, about a legitimate issue that relates specifically to that authority.
For example, in the case of Lori Vallow, who has just been sentenced, no one might ever have known her children were dead, except that her son's grandmother worried about not having skype calls with him. And somewhat freakishly, her home happened to be under surveillance by a private detective, working for a different relative on a different matter... the detective noticed there was no sign of any children and conveyed that information back. It was the relatives who took this information/welfare concern to police, and set the wheels of government authority in motion.
So I think it's appropriate for family and friends of a minor child to be concerned for the child's welfare. Not just in cases of overt child abuse, but also neglect.
Neglect includes 'where a child's environment is injurious to his or her welfare, when a parent/guardian refuses to provide food, shelter, education, or medical treatment'.
Neglect
IMO her media statements show the stepsister offered her own off-grid place, because it 'at least' had shelter and access to grocery stores. So I think there was an understanding of how radical the mother's proposal was.
There's no duty for a relative to report child neglect, but I believe this would have been seen as a legitimate concern if reported to, for example, the child abuse hotline.
IMO, the stepsister made a sincere and concerted effort to prevent this outcome for all 3 of her loved ones. She probably didn't understand or think through the legal principles around child dependency and parental responsibilty, and so probably didn't see it as being her business to come between mother and child.
Making an effort doesn't guarantee the outcome you want. But you can avoid regret and guilt because at least you tried, and did the right thing.
JMO