isn't the whole point of him being a dick that he knew about her feelings for him, and still asked her to die? Like yeah, he absolutely has no obligation to return her feelings, but damn he sucks for doing what he did.
Tbh there's really nothing wrong about him loving the younger sister (except from few icks that it gave me coz of the age...example, when he's a full grown adult and he has a crush on a teenager ... well it's complicated but still gives me an ick from time to time) And yeah, he's such a trash and a jerk when he asked the older sister to take the place of the younger sister knowing full well that she's in love with him and she loves her family and feels indebted to them for adopting her. A Roundabout way OF weaponizing the emotions that she felt is such a trash move. And even after her sacrifices, they can't still respect her deeds. I don't care if your wife might have a mental breakdown but at least, tell the truth about the honorable deed that their aunt did even if ypu have to tell them to keep it a secret from their mom. They chose the easiest and trashiest way to "maintain" the mental stability of the younger sister...and that is to trash of the name of the older sister and brand her as a witch who stole the sacred sword.
Is the 1st love a dick for asking her to die in her sisters place? Yea.
Is he a dick for loving her sister? No, not really.
Just coz the MC likes him doesn't mean he HAS to like her back, we as humans can't control our emotions, just how we can react to them or wtv. If he HAD to chose between MC and the sis, I wouldn't hate on him for choosing the sis [BUT AT LEAST HONOR HER MEMORY GOSH DARN!!!!]