My hero has to kiss her lover at some point, but he is already in another relationship (wtf), so I can't. Every time I try, the lover always reject my sim because he's loyal to his lover (called Viviena -> Idk who is that, some in-game character).
I already tried the Elixir of Allure, but it's not working.
The other idea was to kill her; I tried to search her first, but never find in any room.. And because she is neither related to my 2 playable sim I can't mark her.
I'm afraid to quit the quest, I don't want to lose it. :_)
(it's not a simple love triangle anymore right? xd)

Sounds like he has the chivalrous trait. If you can get him in to cas you can change that trait on him. (You'll need mods). Grim's core has edit in cas in it and Deedawg has a standalone edit in cas mod, if I remember correctly, they should give you access to the traits as well.