Obi-Wan was well aware that Anakin had traveled to Tatooine (and the ostensible reason for his visit and the fact that he'd spent time with his mother before she died as well as the identities of his family) but was unaware of what had happened between Anakin and the Tuskens.
“Anakin, no!” Obi-Wan shouted as Anakin started forward. Anakin almost
hesitated. But Obi-Wan didn’t know about Tatooine and the Tusken
Raiders. Obi-Wan must never know. And the only way to keep him from
seeing Anakin in the same mindless rage was for Anakin to charge now,
while he still had control of himself.
Star Wars: Attack of the Clones - Official Junior Novelisation
and
“On his way to Naboo,” Obi-Wan said. “He is escorting Senator Amidala home.” Anakin had told him of Shmi’s death; that was why he and Padmé had gone to Tatooine, he said. Obi-Wan had talked to Padmé later, and she had explained that Shmi had been kidnapped and killed by Tusken Raiders.
Neither of them had been willing to go into much detail, and from what Obi-Wan knew of the Tusken Raiders, he didn’t blame them. It was no wonder Anakin seemed shaken, if his mother had been tortured and killed. One day, perhaps, Anakin would be willing to tell him the whole story.
Star Wars: Attack of the Clones - Official Junior Novelisation
Notably, not only does Obi-Wan know the identities of Anakin's family, but it would appear that he has intel on them from after the point that Anakin left the planet in the previous film.
“Cliegg Lars still lives on Tatooine, I think-and Anakin’s stepbrother
… Owen, that’s it, and his wife, Beru, still work the moisture farm
outside Mos Eisley …”
“As close to kinfolk as the boy can come,” Yoda said approvingly. “But
Tatooine, not like Alderaan it is-deep in the Outer Rim, a wild and
dangerous planet.”
“Anakin survived it,” Obi-Wan said. “Luke can, too. And I can-well, I
could take him there, and watch over him. Protect him from the worst
of the planet’s dangers, until he can learn to protect himself.”
Revenge of the Sith: Official Novelisation
