Additional District Judge Anil Kumar Agarwal also slapped a fine of Rs 10,000 each.
In July 2009, Seema was beaten to death and her body was disposed off in Dauhari village of Rajpura area in the district, prosecution said.
After hearing the arguments, the judge held Seema's father-in-law Nathu Singh, his wife Jaywani and son Satish guilty.
Seema's husband, Devendra, was minor at the time of incident and his case is pending before juvenile justice board, the prosecution said.