Allen Stewart, Stewart's cousin and adopted brother, was shot and killed Wednesday in Seattle, The Kansas City Star reported on its Web site Saturday.
Kascey Corie McClellan, Jackson's 19-year-old cousin, also died Wednesday after being shot a week ago at an Oklahoma City nightclub.
"We've got a lot going on, and it's not the players' fault that they're distracted. We had two murders this week with immediate family, and I've never coached that before," Jayhawks coach Bill Self said Saturday after his team lost 61-60 at Oklahoma State. "I don't know the coaching manual on some of that stuff.
"But still today, we were all here, so if you're here you've got to perform. We just didn't do that."
Jackson led Kansas (No. 5 ESPN/USA Today, No. 4 AP) with 16 points and 10 rebounds, picking up only his second double-double of the season.
"It didn't make it hard to play at all. It just made me focus even harder and try to play hard for my teammates," Jackson said. "Sometimes it doesn't work out that way."
Stewart played 11 minutes, about 3½ more than his average in conference play, and missed his only shot. He had one rebound.
"I think they're doing OK, but nobody wants to ever go through something like that," Self said. "But it happened, and the best they can do is deal with it, and I think they've actually done a pretty decent job as far as allowing people to help them and deal with some things that are totally senseless."
Authorities said Allen Stewart, 21, was sitting at a stoplight when a man in another vehicle pulled up and shot him. Rodrick Stewart's family adopted Allen in the sixth grade as his mother was in prison and his father was absent. He and Rodrick graduated high school at the same time.
Bull Stewart, Rodrick's father, said the incident has been tough for his son, who traveled to Stillwater, Okla., on Saturday for the fourth-ranked Jayhawks game against Oklahoma State.
"He wanted to come [home] immediately, but I told him there was nothing he could do here right now," Bull Stewart said, adding that his son planned to fly home after the game and would miss Kansas' game with Iowa State on Wednesday.
Jackson has had to deal with several misfortunes recently. He lost his grandmother, Evon, in a car accident in 2005, and his uncle was beaten to death with a hammer. McClellan is the second of Jackson's cousins to die in a shooting.
"He's keeping it together," said Jackson's mother, Shawn. "The family is trying to be really positive for him."
The family is trying to plan McClellan's funeral around Jackson's basketball obligations and about 30 friends and family members planned to attend Saturday's game in support.