I find that unlikely. I mean, what if there are no brothers? If, for sake of argument we say there are always exactly two children per family (approximation for stable population size and if some brothers die or move), there's about a 50:50 chance that a woman won't even have a brother to help raise the children.

Seems super unlikely to be the "most common" (by number of societies) arrangement when it only can work like 50% of the time in a society that DOES decide to do it. Was your Anthropology professor pulling your leg?