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.
I doubt that it was genuinely a one-time transgression; he and the neighbor lady probably engaged in multiple acts of intercourse; while it's possible to become pregnant through a single act, it is still rather unlikely.
And there may have been still other "neighbor ladies" with whom your father consorted.
[...] and he and the mother both were never sure [...]
Or simply didn't care.
I sincerely sympathize with you. (The "babysitting" part of your story is especially poignant.) You bear none of the guilt in this case - but have been forced to deal with the long-term consequences.
Regards,