It said in the promo that she have a brother who looks just like the duke but the duke nev...

Promo was for the novel so it’s most likely that the actual manhwa isnt following the novel which is why there’s prolly no brother

I saw the brother in the 2nd novel cover maybe he’ll be revealed later

or maybe her brother is actually the Duke's dead brother's child? lmao, just like Hugo and Damien in Lucia ╮(￣▽￣)╭