The court in Bhandara, a town some 750 kilometres northeast of Mumbai, also sentenced two others to life in prison for their roles in the 2006 killings. Three others were acquitted.
Prosecutors say the violence was sparked by a land dispute between members of higher castes and the family from the lowest caste, known as Dalits.
The woman and children were beaten to death with sticks, iron rods and chains, and their bodies dumped in a canal.
The woman's husband was the only survivor in the attack on the family.
Discrimination based on caste - a complex social system that divides people into hundreds of groups defined by livelihood, class and ethnicity - has been outlawed for more than half a century in India.
But it remains pervasive, particularly in rural India where people from lower castes are often barred from using upper-caste drinking wells, kept out of temples and denied spots in village schools.
Ignoring the prohibitions is often met with violence.
The Bhandara homicides drew widespread attention after complaints from Dalit groups about the slow pace of the investigation.
"This is a key judgment because it sends a very strong message that brutality, especially to low castes, will be dealt with very strictly," prosecutor Ujjwal Nikam said.
Defence lawyer Sudeep Jaiswal said his clients would appeal the verdict.
"The death penalty is for the rarest of the rare cases and certainly does not apply here," he said.