>>2044350As I recall, the situation was complicated by the "canon immigrant" scenario from the film--the Gate of Judgment. Amid this whole sneaky deal between Bell and Keiichi, Hild decided to stir the pot staging a coup in Nifelheim against herself (it makes sense in context) by the top demons under her which, besides its intended purpose, would force the couple to confront the awkward question.
The reasoning for that, besides Bell being the half-sister of her daughter Urd, was that a much-younger Hild crossed the Gate of Judgment with a resident of Heaven and failed the miscegenation test that became the norm after the Lake Goddess's relationship--with Urd's father (the one she shares with Bell). Ergo, the end of their relationship and the beginning of the Almighty's relationship with Bell's mother--much to Hild's chagrin. In the manga Hild is generally fond of the couple, but like Urd has questioned their lack of physical intimacy, and decided to act on this. And that eventually gets Bell's mother involved (for her first appearance in the series).
At least, that's how I recall the situation playing out--I may've gotten certain details wrong.