I know there’s a public debate whether the girl is Kaname’s or Zero’s.
The translation said “my other father” instead of “ours”.
The Japanese, as ambiguous as the language is, never really specified which.
The line says「 母さんからもう一人の父さんに」”from Mother to the other Father”, without a subject.
Usually, when a person doesn’t specify a subject in the sentence and without supporting verb which indicates otherwise, it means “I”. So yes, “the other father” could be exclusively the longhaired one’s.
However, in Japanese culture, one doesn’t need to address “us” when the people involved are insiders, let alone a family member. And thus “the other father” could be inclusive to the other son/daughter (I haven’t determined the gender) as well.
Conclusion? Just work on your bias here. It doesn’t matter which.
Personally, I’d like to think they are both fathered by Zero. “The other father” means how Kaname is accepted by the Kiryuus as the one precious to their mother
and father, with huge roles in their lives. Admittance as a part of the family.
On the other hand, I also like the other opinion in a forum that the girl’s father is Kaname, to cater both sides’ fans. It doesn’t matter which, really, just go along how your heart believes it to be.