I think you guys are all wrong on this one. It's not about wether or not Michael killed all those people. She definitely has doubts but she decides to beleive him. How could she be sure he ordered the killings?

The signification of the last scene is that when she turns around and she sees the capos kissing the Don, she is confronted with the reality that these men are more intimate with her husband then she is. She doesn't know if he ordered the killings but she knows for sure that THEY know. There are something between them, the 'sicilian thing', that she is not part of.

This is, I think, what the last scene meant.

Paolo Violi.