He’s treating her well cause he understood why she treated him the why she did if that makes sense, and in their first life they started to mound their relationship and that’s why the aunt started to hate the duke’s family. Also big spoiler but Their actual blood related like Lucas and his uncle are not blood related at all.. some goes for the duke and duchess. So Luca’s only blood related family is the aunt. The princess cheated on the duke and had Jones. So basically Lucas and the aunt are at strangers house and Lucas knows but the aunt doesn’t
Also for more good explanation go here
http://www.mangago.zone/home/mangatopic/11833919/
I think that Luca is the only one who went back in time (I could be wrong here), and the reason why he's cold to his uncle, is that he cares about him, and for this reason decided to keep his distance from him, to avoid uncle sacrificing his life fpr his sake