I think people are missing the true reason behind the MC feeling so guilty for helping his sister win. In a later panel, we see a young MC sitting in front of a piano with the caption reading "because I experienced the emptiness of hard work not paying off." With this context its pretty easy to fill in the blanks.
MC busted his ass in a competition and lost, and his parents berated him for losing, even though he performed well, and by helping his sister win the election on a whim of "might as well", he pushed a this one girl (and likely more) into that same position of their hard work and being second place meaning nothing to the family all because they didnt win. So MC, knowing that feeling directly, chooses to never push anyone into that position again, even unintentionally.
At least thats how I understood it and was surprised at how many people seem to not realize it.