Will the shocking twist be that what Susan suggests in panel three is totally the right answer?! Probably not, what with me asking that and such, BUT STAY TUNED TO FIND OUT!
What Tedd said waaaaaay back in the days of 2012:
"...having spells and/or an inborn talent for magic has an impact on a person's physiology. Exactly how varies."
As for why Justin's apologizing for accidentally strengthening the half-sister argument, recall how Susan took the half-sister suggestion.