Tom was smart enough to figure things out. I'm sure he didn't accept the accident explanation of Fredo's death. Like Vito, he would never had murdered a member of his family. But his loyalty to Vito's memory probably kept him from overtly reacting to Fredo's murder because that might put Michael in danger. In fact. I'll bet that that his turning to amistress was an effort by Tom to ease the trmendous stress he was feeling over Fredo's murder and his denoument as Consigliere.

"Generosity. That was my first mistake."
"Experience must be our only guide; reason may mislead us."
"Instagram is Twitter for people who can't read."