He testified in a nearly empty courtroom, missing two Spurs’ road games on the West Coast. The Kerrs learned two months later that they had won the suit — millions of dollars that they may never see. But money was never the point.

“It provides a structure to enable people to channel their feelings through justice and the rules of law, rather than become vigilantes,” his sister, who is now known as Susan van de Ven, said in a phone interview from England, where she is involved in politics as a county councillor. “It gives a very focused approach to people who are rightly and insanely aggrieved. That’s the kind of culture we should have. We shouldn’t be responding with violence. I’m sure that’s why Steve talked about guns. It’s all related, isn’t it?”

Her book detailed the family’s experience with the lawsuit.

The night before the Warriors visited President Obama to celebrate their 2015 N.B.A. championship, Steve had dinner with Andrew, who works for an architectural design and residential builder in Washington. They discussed what Steve might say to the president. Andrew recommended complimenting him on his efforts toward gun control. Kerr did.

In June, at the end of a podcast with Bay Area sports columnist Tim Kawakami, Kerr asked if he could raise one more topic. Our government is “insane,” he said, not to adopt stronger background checks on guns that most Americans agree upon.

“As somebody who has had a family member shot and killed, it just devastates me every time I read about this stuff, like what happened in Orlando,” Kerr said, referring to the June massacre at a Florida nightclub. “And then it’s even more devastating to see the government just cowing to the N.R.A. and going to this totally outdated Bill of Rights, right to bear arms. If you want to own a musket, fine. But come on.”

Since then, Kerr has become a go-to voice in sports for matters of bigger meaning. It surprises his family in some ways, knowing that he was probably the quietest of the siblings as a child.

“He’s carrying around the family business in another discipline,” said John Kerr, a professor of community sustainability at Michigan State. “There was no way he would do anything for a living that didn’t involve sports. No way. And now that he’s at the pro level, he has the opportunity to speak out. He’s smart enough to realize he can do it.”