"You're not at all suitable for Miles."
Janus Stanhope certainly didn't keep his opinions to himself--he wanted Georgina to know he didn't approve of her engagement to his younger brother.
Under the circumstances, Georgina couldn't enlighten him. In any case, he was so arrogant she felt he deserved to be fooled!
Georgina was only pretending to be Miles's fiancee--to help him out of a bind. But now she found herself in a mess. For Georgina found Janus far too attractive for her own peace of mind!