Exactly!
If this were some total stranger, you would be justified in being skeptical - but if turns out that you actually babysat this person, then the reality is: Your father had sexual relations with that neighbor woman.
Does this change your opinion of your father? I mean: The fact that he probably knew it was his child (that they were his children?), and yet abandoned them?
Regards,
This does not really much change my opinion of my father, as he more-or-less abandoned the five of us, when I was about 10 or 11. He was not a good parent. Late in his life, he chalked it up to not having “emotional intelligence”. He was very, very smart in other ways.
I don’t know if he knew about the other child, but seems like he could have or should have. But in my mind, I am giving the benefit of the doubt that maybe it was a one-time transgression, and he and the mother both were never sure, and chose to live their lives as if the child was from the mother’s husband.