I mean, I don't really get why he wouldn't say "I'm your brother, we haven't forgotten you and your dad has been looking for you both all this time, and we're totally going to save your mom" :/
There literally isn't a single reason why he has to hide his identity from her? This is getting really convoluted. Why does his whole family have to start off hating him like some sort of heroine capture starting point? His dad has a mental break on him, his little sister Norn got a bad impression cause he punched his drunk ass dad, and now his little half sister has to assume he doesn't care about her because...reasons? No doubt Lilia is going to go off on him for not taking her daughter and running like she asked. Just tell her dude.
I'm half expecting his mom to be some sort of emotionless killing machine that hates his guts the moment they discover her, or has completely forgotten who he is. He can't have a single moment where he sees someone he loves and they just have a completely normal reaction to him.