If Tessio was a traitor to the family, why he didn't reveal Michael's plan to kill Sollozzo?
Because he hadn't turned on the Family at the point.
This is, perhaps, better explained in the novel as the movie does not really have time to cover the whole issue.
Tessio was loyal to Don Vito who was still alive at that point and although he supported Sonny he thought more of Michael.
It's only after Vito retires/passes and Michael takes over that Tessio starts resenting Michael for not letting him retaliate against against rival Don Emilio Barzini as the Barzini family chips away at his bailiwick in Brooklyn.
In the end he betrays Michael for his own benefit
“He’s the most intelligent,” Michael said. “And he did what seems to him to be the smart thing. He sets me up for the hit by Barzini and inherits the Corleone Family. He sticks with me and he gets wiped out; he’s figuring I can’t win.”
The Godfather - Mario Puzo