What Arthur said was only half the truth.
Because it was indeed Gianna who had sold Evelyn to the slave trader. Indeed, Evelyn realized very soon after Gianna had left her that she was liked by Gianna only because of Oliver.
As long as Oliver was alive, Gianna liked her a lot thinking that she would take care of Oliver.
But the reason behind selling her to a slave trader was because Gianna wanted to avenge Oliver's death from Arthur. She knew that Arthur liked Evelyn and hence killed Oliver.
So she attacked the person most dear to Arthur.
Hence, Arthur's lies were half-truths.
Still, Evelyn couldn't believe how easily Arthur put all the blame on Gianna. Did he need a reminder that he killed his brother Oliver?
However, Evelyn was also aware that she didn't have any evidence, and without evidence, she couldn't speak again Arthur.
But she still could do something.