She is being too conceited. Would he really be happy without her, would he want to be a knight? What about Luna, without all these events, she would be probably in a ditch, dead or used for experiments/war. Sister seems to me almost like a housewife, not really going out much, but taking care of their home and supporting him where she can. If she could just stop blaming herself and start to enjoy life instead, I'm sure MC would be happy as well.
Or if she really wants to pay him back, she would orchestrate a perfect female party for him. There is Luna for magic, his knight friend for physical attack, and his healing buddy from the old hero party. If she can make them all come together, creating a harem of girls, that would be the best sister he could wish for.
Unfortunately, I don't see this development any time soon. They would drag this on probably with MC being siscon for a long time and nothing will happen with other girls.