"Back then" you would try and kill your oldest brother to inherit the 2 chickens and a goat your parents would leave to him.

Read no further than the histories of nobles and their feuds and murders to see how bad the best families were.

The nobles were the richest families, but not necessarily the best by any other measure.

Best by food availability, shelter, labor.

Given that even in good years a large portion of the population was malnourished in whatever time period you want to pick before the middle of the 20th century, the rich were always better off than the poor and rich families were better than poor ones.

Right, all those things are different ways of saying wealth.

The implication made uphtread is that the rich were best in terms of (lower) intrafamilial conflict. One might assume that wealth produced that effect, one might also assume that the rich were more dependent on social status and property for support, while the poor were more dependent on having close relations who could give aid and lacked assets to fight over, thus leading to less intrafamilial conflict than among the rich.