Ok guys, here it is. Pretty simple really.
The Mafia is "The Family" and "La Cosa Nostra" (lit. "Our Thing"). It's both a metaphor for the extended social network in general, and an instance thereof.
The social anthropologist Diego Gambetta has claimed that the organised crime networks of southern Italy came about as a response to the inadequacy of the legitimate governments of Sicily and Calabria, imposed on the southern public from above by the north, often themselves rife with corruption and despising the peasant population. In order to effect a collective social strength these southern communities had to create their own, de facto governments based on omerta and a complex system of loyalty and favours, a system that was exported wholesale to the US into impoverished, Catholic migrant communities.
One of the consequences of late capitalism in countries such as the US, based around home ownership and lifetime debt, has been the disintegration of the extended social network. As family sizes shrink and the institutionalisation of family members who are no longer economically productive becomes a norm, the family sphere shrinks to mother, father and children within the suburban home. Where the "frontier of compassion" used to be the boundary of the extended network - whether it be village, parish, clan or crime organisation - now no care is given to those outside the nuclei of that network, the family units themselves.
There is therefore a fundamental barbarism that commences at the boundary of the nuclear family. Beyond this boundary no true charity exists, no help is given without contingent expectations. Any disproportionate atrocity is feasible to protect the wealth and welfare of the family. This permits the dispossession and humiliation of those who in theory should be near and dear, including one's parents and siblings.
I believe the Sopranos, to the extent that it is about anything, is about reflecting the essential viciousness and isolation that is enforced upon all suburban homeowners by the parameters of debt and peer acceptance. Alongside the callous, avoidable murders of prominent characters - friends and family - and the humiliation of Junior, we see the obsession of Tony and Carmela with the progress of their offspring, and Carmela's worries about building and renovating the family home. The gangster milieu permits the actualisation of the brutality that is implicit in all suburban life.
The show concludes with a tableau that deeply expresses this contrast between the suburban siege mentality (the family literally walled in by the diner booth, and viewed through glass) and the attendant orgy of consumption.
