>>725243394
>If only he would be able to pass the treasure that he got from his father onto his daughter –Tokiomi thought sadly.
>However, that was already impossible to be achieved.
>For Rin and Sakura, there were no choices for them in the first place.
>One of them has all elements, having five multiple elements as her alignment. The other has no elemental alignments, having Imaginary Numbers only. Both sisters have a rare potential that can be equated to miracles. This had surpassed the limits of so-called natural talents or inborn skill; it is almost like a curse.
>A magical nature would equally gather magical powers to it. Prominent people who are far outside the rules inevitably ‘gather’ equally extraordinary experiences. This can not be controlled by the person’s own will. There is only one way to deal with this kind of a destiny – consciously walk away from the rules yourself.

>Apart from understanding and practicing the way of magecraft themselves, there are no other ways to deal with the magical powers hidden in the blood of Tokiomi’s daughters. Moreover, the protection of the Tosaka house can only be endowed on one of them. This fact tormented Tokiomi for a long time. The one who did not become the inheritor would get mired in all kinds of odd evens due to her own blood, and trouble wound find her. If the Association found this kind of ‘ordinary humans’, those guys would definitely gladly put her in formaldehyde as a specimen in the name of ‘protection’.
>Precisely because of that, it was nothing better than a godsend for the Mato house to hope to have Sakura as their adopted daughter. He had obtained the way to have both his beloved daughters inherit first-class magecraft, unconstrained by their bloodline’s consequence, and carve out their own lives. At that time, Tokiomi could be said to be freed from the heavy burden of being a father.

Here's some more context from the Fate/Hollow Ataraxia VN.