It’s good advice, but the reason he didn’t tell his sister is that he knows what would happen if he did and doesn’t want to worry her.
He doesn’t confess to Saemi because he’s afraid of rejection and possibly ruining their friendship.
He didn’t tell Lala because—why would he? What does that have to do with her? I could say the same about him being nosy about Lala’s personal problems, but then we wouldn’t have a story about the MC being the good guy who saves girls from danger.