I love how elegantly the problem was solved.
Indeed, the whole issue of marriage between parent and child is a problem only if you think about it in Westerner terms (going to a church, asking for divine blessings, all that stuff). If you think about it in Japanese terms, marriage is not different from adoption: someone who wasn't in your family register goes to the city hall with you and enters your family register — and from that day on, you're family. Thus there's no need for a family member (a child, a sibling) to marry you: you're already like that since the day you were born. Brilliant!