With how their dad doesn't treat him differently (the whole "cursed" hair thing) along with how his brother acted in the past, I'm guessing that his brother is enacting a similar plan (i.e. "act badly to elevate him and besides, he actually loves my fiance as a person")
Looking at Yuuki's screen though, he doesn't have that skill...
So either a) Alice wished to be like her Lord and Nos wished to be human while dying, thereby granting them immortality/mortality respectively, or b) Yuuki has yet to re-unlock his immortality...