when ML ask MC brother about love he is really happy since ML got himself a woman and he can protect his sister since there's no way MC can be acquainted with psycho like him . So he support him and encourage him to pursued the lucky lady... Not knowing the said lady is his sister and digging his own grave
a little bit of their conversation
brother : then let me ask. what is the reason for you to not want to killed them?
ML: everything. i like everything.
ML: i probably really like her to the point i want to see her forever.
brother: then what the reason thats makes you want to killed them?
ML: when i dont want to killed her. (he want to kill her cause he dosnt want to kill her (he's confused of his feeling xD
brother : <this discussion only goes in circle, like always i dont know what this beast is thinking> then what if you cant meet them again?
ML: no way thats happening, no.
MSB: <...isnt this a good chance?> then can you tell me what do want to do to them?
ML: I WANT TO HUG HER
ML: I WANT TO HUG HER WHILE ROLLING AROUND AND SMEAR HER WITH MY SALIVA?
ML: I WANT TO BITE HER UNTIL SHE BLEED AND LICK HER UNTIL HER FLESH TORN APART?
ML: AND TH-
brother: STOP! THIS IS FAMILY FRIENDLY WEBTOON YOU KNOW!
ML: .... <death glare. you are the one who ask>
brother: so that person is a lady? in other words... you miss her? cant stop thinking about her and it made you happy? and you want to touch her and kiss her right?
ML: yes 3x
brother: but when you want to do it, it didnt happen and you are disappointed right?
ML: ....how do you know about that?
brother: <its finally happening! to this beast!> congratulations your highness, you are in love.
credit : https://forum.novelupdates.com/threads/beast-with-flowers-kr-manhwa-webcomic.104387/#post-5580353