These people don't seem to have an adequate sense of "what if".
It feels to me like they are starting with a basis in the Continuum Concept, which was a book from a thesis written by a childfree graduate student who studied a native culture in South America. The concept that people who read the book have come up with is called "benign neglect", but trust me, it has NOTHING to do with actual neglect. It has to do with...instead of sitting there "playing" with your child for hours on end, you engage in the normal everyday living aspects, with your child around. So you do the dishes while baby bangs pots and pans (I never did that with DS, ears are too sensitive, and ultimately b/c of his personality he jsut wasn't allowed in the kitchen anymore), you do laundry with baby on your back, etc etc. It comes from the idea that the tribal people *have to* do their daily work or they don't eat/drink, and they don't have time to play directly with the kids, and the kids need to learn the work as well, so the kids play and engage around the work of the tribe.
However, if that's where they started, they've gone off the deep end and need to be pulled back.
I'm very glad your brother listened to your mom about the leaving him alone thing. I think that was a good place to start, and hopefully they really *got* that their actions have been wrong and they will strive to change.
I think Molly is exactly right, and to top it off the have bought into the "it takes a village" mentality as shown at the party.I do think within this same belief system your mother as matriarch is the exact right person to talk to them.
In the meantime it is not incompatible to call CPS, as most likely it will take multiple calls over time for them to even do an investigation. A call to CPS is not a magic bullet.......it is a state agency where things move slowly per their protocols. If your brother straightens out, no harm no foul.....but if not you all ready have the first complaint registered.