Sujon was the fifth member of the family to succumb to burn injuries
Chattogram Division's Speedy Trial Tribunal Acting Judge Md Abdul Halim passed the verdict on Monday.
Three others were acquitted in the case, said Public Prosecutor Ayub Khan.
Trader Jahangir Alam was hacked to death in the Gouripur village in Cumilla's Daudkandi Upazilla on Dec 1, 2013, he told bdnews24.com.
Jahangir's father Fazar Ali started a case over the incident which went to trial in Chattogram's Speedy Trial Tribunal two years later.