The two regularly bullied and abused Yuriko. Both Eriko and her boyfriend were on welfare; but, teenage Yuriko had to get a part-time job to cover entertainment expenses, like buying a game console (news reports don't specify which).
On the day 17 year-old Yuriko was murdered at their home, her older sister and Umezaki became incensed after Yuriko didn't practice playing video games—she skipped her practice session, apparently. Other things that made them mad was that she didn't take out the trash. They beat Yuriko with brass knuckles, their fists, and a chair until she died.
A jury of their peers—a relatively new addition to the Japanese legal system, judged the two. The jury system was reintroduced (it existed before World War II) in hopes of making rulings fairer. Nothing about this, however, seems fair.