Look at this S! Cupid's Prank - Vol. 10 Ch. 103 - My Responsibility

[MangaDex]

 

[etriuswimbleton]

Okay his Dad is cool. Like he may look strict but seems wise and actually kind of caring. He also seems to know more about the birthmark than he lets on. It must be a history in the family tree or something. He may have a way of giving Rin an "Out" so to speak. The dad didn't even question the fact a woman was brandishing a knife at him while they're both kind of naked.

so Dad confirmed that the Mom WASNT affected by the birthmark that way? so if its your own blood relatives it doesnt work?