He's lying all the time. He clearly wanted to take Catherine away, and lied about having seen the Tilneys going elsewhere.

I am not sure about 'and the brother'. Do you mean why James didn't say otherwise?

Maybe James thinks JT had seen what he didn't.

Anyway, JT shuts him up if he tries to say anything to the contrary. He is rather dominated by JT or lets him dominate because of Isabella. Oh, what some will go through for the sake of love.

This line rather sums up the attitude;

“I doubt our being able to do so much,” said Morland.