great issue to raise, OP
some things to consider:
we dont know a whole lot about the families' dynamics at any point prior to the setting of the drama.
clearly, the first mother was also a malevolent person, and must have played a part in the estrangement of the boys, if not also the tensions & conflicts leading to the father's rupture with her and her sons.
the mother of the second family was clearly a kinder and more well-loved (and cared for) person, in the center of her family.
the only things i can inject into the situation were that the older family were left dangling (none of the older boys had any skills - an encapsulation of their abandonment, impoverishment, losses), while the younger boys had property, a family tradition of nurturing, a loving & loved mother.
beyond that, its hard to surmise, in my view of the situation.
the younger family was advantaged, relative to the older family. hence the essential injustice leading to conflict. the dispossessed demanded to be heard.
that hearing was enough in the flow of provocations (far, we all understand, from the first) to start the ball rolling.
all that said, the dynamics of the town might be understood to have concluded that the first family, including the mother were 'bad apples/white trash', while the second family were people of a higher quality.
just another aspect of the film that is left to our speculation, really. there's no reason to expect/assume that every town acts rationally, or in perfect accordance with our expectations. add to the fact that there's so much, again, that we don't know about the story, its hard to draw conclusions.