It had nothing to do with blood relation. Omichi is the base because she physically wrote his novel for him when he became blind. The Eight Dog Chronicles literally could not have existed without her, as it was her hands that put down his dictation to paper. Baikin was the mind that crafted the novel, and she was the hands (body) that actually wrote it.

Oei is the base for her father because she (allegedly) crafted some of her own work under his name (and thus, again allegedly, some of the work credited to him is actually hers), not because she is his biological daughter. He had another daughter (the younger blind one from their Interlude), but his other daughter can't be his base as she has no connection to his actual work as an artist, which is what qualified him as a Heroic Spirit.

Servants don't manifest in the bodies of family members unless the family in question has a very strong connection to their legend/accomplishments.

For example, if the family member actually performed the feat themselves despite not receiving the credit, or if their aid was indispensible in accomplishing the feat that qualified the Servant for the Throne, like in the example of Baikin and Omichi. Baikin literally could not have completed his most famous work without Omichi. If someone else had physically written the novel for him, they could be his base.

OmniGManJr said:

It had nothing to do with blood relation. Omichi is the base because she physically wrote his novel for him when he became blind. The Eight Dog Chronicles literally could not have existed without her, as it was her hands that put down his dictation to paper. Baikin was the mind that crafted the novel, and she was the hands (body) that actually wrote it.

Oei is the base for her father because she (allegedly) crafted some of her own work under his name (and thus, again allegedly, some of the work credited to him is actually hers), not because she is his biological daughter. He had another daughter (the younger blind one from their Interlude), but his other daughter can't be his base as she has no connection to his actual work as an artist, which is what qualified him as a Heroic Spirit.

Servants don't manifest in the bodies of family members unless the family in question has a very strong connection to their legend/accomplishments.

For example, if the family member actually performed the feat themselves despite not receiving the credit, or if their aid was indispensible in accomplishing the feat that qualified the Servant for the Throne, like in the example of Baikin and Omichi. Baikin literally could not have completed his most famous work without Omichi. If someone else had physically written the novel for him, they could be his base.

I think the artist's comment means base of design, so they are correct - Omichi has no blood relation to Bakin, so they look nothing alike, so using her Servant-Bakin design as a base for his individual design is incorrect.
Both father and daughter Katsushikas do share blood relation, so it makes sense to base Hokusai's design of Oei's, because it makes sense for them to look alike. In fact, closest thing we have to official human design of Hokusai the Elder, an art by Hokusai's official artist looks very much like Oei.
Of course, there are family members who look nothing alike, and total strangers who look very similar, even in Fate design-wise - you wouldn't suspect that Hector and Paris are full-blooded brothers (perhaps they have that anime-genetics thing where one child is a near-clone of the mother and other one of the father, or Apollo is to blame), or that Okita, her Alter and Lakshmibai are completely unrelated to the rest of Saberfaces and their design is a gag (Nero and Artoria sorta have a shared ancestor, Jeanne is from very roughly the same area and may be descended from Artoria's relatives, Morgan and Mordred are Artoria's closest family and even then an appeal to some distant shared ancestor could be made for Lakshmibai).

nightdreamer said:
Of course, there are family members who look nothing alike, and total strangers who look very similar, even in Fate design-wise - you wouldn't suspect that Hector and Paris are full-blooded brothers (perhaps they have that anime-genetics)

Apollo (the sheep with Paris) is responsible since aside from being as bad a skirt chaser as his father, Zeus, he's also apparently a shotacon/pedophile since he forced Paris' saint graph to materialize him as a child. (with mint green hair and sheep attributes)