>a mother who (as she tells us) encourages him/says she loves him at least three times a day etc. Kevin had no reason to do what he did other than he was born a sociopath.

You obviously didn't read the book.

The mother was abusive and neglectful.

She played a significant role in Kevin's development.

>a dad who went above and beyond (the absence of a father figure is usually a symptom of the problem)

The dad was a part of the problem as well. He coddled Kevin and never tried to seek help for his mental illness.

Both of these parents were horrible and should've never had children in the first place.

>His sister was brought up in the exact same conditions and was a loveable child.

Abusive families often use one child as a scapegoat to prevent siblings from feeling any sense of solidarity against the abusers.