The oldest son of Demetrius II and Cleopatra Thea was just a child when his father was taken hostage by the Parthians (139 BCE), so his mother married her husband's younger brother, Antiochus VII. When Antiochus invaded Parthia (130 BCE) to reclaim lost Seleucid territory, the Parthians released Demetrius to challenge his brother for the Seleucid throne. Within months Antiochus fell in battle (129 BCE), clearing the way for Demetrius to reclaim both wife & kingdom. But he proved incompetent in ruling either. In 126 BCE Cleopatra locked the gates of Damascus against him after he lost a battle & ordered his murder weeks later. Though still a youth, Seleucus was installed as her co-regent -- hence his byname.  Yet soon after she engineered his death as well, for reasons not entirely clear.* But she may have thought Demetrius' younger son, Antiochus VIII, a more compliant co-regent.

* Writing almost three centuries later, the Roman historian Appian of Alexandria claims Cleopatra personally killed Seleucus with an arrow, fearing he might try to avenge his father. Yet both allegations are dubious since there is no evidence that Cleopatra was trained in archery & she was perfectly content to make Seleucus' sibling her co-regent.  

Reference: Appian, History of Rome: Syrian Wars 68-69. *                

Other resource on line:

• Seleucus V Philometor - brief entry in Wikipedia's web.