I would probably view the relationship between Malcanthet and her tiefling prodigy being a mixture of bordello matron and distant grandmother. As a fiend, Malcanthet has little to no care or concern about her tiefling descendants. They are her living tools to perform the work that she or her full-blooded succubi cannot accomplish. Most of these tieflings act as her eyes and ears on the Prime Material Plane, where fiends are more easily identified and defeated via magic. Paladins can detect fiends, but tieflings don't register to such scrutiny. Plus, protection for evil and banishment spells can thwart a fiend, but they are nearly pointless against a tiefling with such a small amount of fiendish blood in their veins.

As for some sort of origin story for a Malcanthet tiefling bloodline:

It was centuries ago, when a group of bedraggled priestesses approached a lone monastery that stood in the high mountains. The friars that resided in the monastery lived an isolated life to help them achieve a state of purity of mind and body. However, while their vows prevented the friars from indulging in the vices of the world, it also prevented them from turning away the group of women in need. They opened the doors of the monastery to provide food and shelter to the priestesses, who ended up being the friars' damnation over the long term.

The priestesses were actually succubi, and their head priestess was Malcanthet, Queen of Succibi. The fiends repaid the monks with their amorous attentions until the men eventually succumbed and forsook their vows of chastity and celibacy. Over the many years, half-blooded prodigy were born from the unholy unions between succubi and humans until eventually Malcanthet created a line of tieflings tied to her succubus legacy. Her work at the monetary done, Malcanthet returned to the Abyss, but not before she instructed her tiefling heirs on what their missions should be...