I agree with Lilo that Michael lacked the fraternal/paternal warmth that might have kept him more involved with Fredo. As far as he was concerned, giving Fredo chores to do constituted "takeing care" of him. I do agree with dt that, given Fredo's near-treachery with Moe Green, he should have kept more of an eye on him.

Ntra la porta tua lu sangu � sparsu,
E nun me mporta si ce muoru accisu...
E s'iddu muoru e vaju mparadisu
Si nun ce truovo a ttia, mancu ce trasu.