That little speech Michael gave Tom before leaving was certainly designed to play to Tom's weak points because at that point in time, Michael had no choice but to trust him. It wasn't out of love but cold calculation. I don't think Michael believed anything he said about brotherhood with Tom. The gratuitously cold way he treated Tom upon his return was I think, his true feeling towards Tom.
There's no question that Michael's homily reeked of insincerity and was designed to play Tom like a violin ("I always wanted to be thought of as a brother by you," clearly indicating that Tom
didn't believe Michael thought of him as a brother). Perhaps Michael didn't have any choice but Tom at that point. But, Michael might have thought that, because of his coldness to Tom, he could have been in on the Tahoe shooting as vengeance. Then again, Michael probably figured (correctly) that Tom's loyalty would override any plots for vengeance. The family was all he had in life.
