Tibet is the extreme example of grinding malthusian conditions. 1/3rd of men were in monasteries, which of course did "spiritual" military expertise to be ready for defence, but cheaply idle in other times, since there never was enough food to launch an invasion
"Brothers sharing a wife" is again, probably the cheapest possible family arrangement
In parts of rural Tibet, brothers traditionally married the same woman, so the family’s land and herds wouldn’t get split up. Some villages still do this today.