The lives of Paul and Keith Caneiro had always been entwined. Born just a year apart, the brothers created businesses together, moved to the New Jersey suburbs to raise families, and appeared to be savoring the fruits of their shared labor, from handsome homes to luxury cars.

But lately, there had been signs that the relationship was unraveling, and on Thursday, Paul J. Caneiro, the older of the two, was charged with killing his brother, his sister-in-law and their two young children and setting fire to their $1.6 million home in upscale Colts Neck, N.J., just days before Thanksgiving.

The killings roiled this quiet, rural corner of Monmouth County, where there were 12 homicides last year, and shocked relatives and friends who knew the brothers to have been close.

“This one is the most brutal case that I’ve seen in my experience here,” Christopher J. Gramiccioni, the Monmouth County prosecutor, said in announcing the charges.