In the novel version, their 5-YEAR-OLD SON declares his own mother to be his fated partner. Shortly after about 10-ish months, the little sister is born and somehow same son changes his fated partner to his own little sister. Bertia-mama doesn't see the problem and only sees that the older brother loves the little sister as family, only the dad knows what the true meaning is.
Later on, the little sister wants to have a romantic love, but her older brother keeps interfering with that. The brother won't accept anyone who can't defeat him, and the sister says that the only person who can is their own father. Bertia just thinks they get along well.