Did you ever wonder what happened to undrafted journeyman Jamel Thomas? No? Who the fuck is Jamel Thomas, you say? Well he was Sebastian Telfair's older half-brother which makes him Marbury's cousin. Now, according to Thomas, the two no longer have a relationship.
In 1999, Thomas believed he was going to be traded from the Trail Blazers to the Timberwolves to join his cousin on a team that went on to make the postseason. He claims Kevin Garnett pulled him aside and told him that Stephon and himself lobbied for his addition in the form of a cushy bench role. But according to Garnett, things went wrong due to Marbury's abrasive approach to management.
Marbury confronted the T-Wolves brass about making as much as Garnett and when he was told they lacked the funds he demanded a trade. At that point the front office for Minnesota said something along the lines of "Fuck this shit.," and wanted nothing more to do with Marbury or his extended family by proxy. Thomas played 12 games in the league over the next two seasons and then went on to play for 13 teams spanning four countries over the next decade.
In fairness to Marbury, the NBA isn't a career where a good word from a friend can get you an entry-level opportunity; but any public rift between family belongs on this list.