What did Ethan want to tell Martin the first time?

At the end of the movie, Ethan tells Martin that Scar and the Comanches had killed and scalped Martin's mother. But it doesn't seem adequate that this was what he wanted to tell him earlier in the movie. He had just been arguing with Martin trying to persuade him to not tag along on his quest for Debbie. Following the first shootout, Ethan makes it clear to everyone that he doesn't want anyone going along, and I assume that includes Martin. Ethan tells Martin that Debbie is not Martin's blood kin and therefore he should have no motivation to follow. After the argument, Ethan says there's something he wanted Martin to know, but Martin doesn't want to hear it. But if Martin knew the Comanches were responsible for the death of his mother, he surely would have motivation to go along with Ethan. So it must have been something else Ethan had intended to say?