WMG / There Will Be Blood

Daniel Plainview has a younger brother who goes by the name of Byron Hadley.
  • The time period makes sense. Hadley is in his late 40s around 1949 when the scene takes place where he relates his inheritance from a dead brother who got rich in the oil business. Daniel Plainview stated that he had a family back east that he effectively abandoned. He started drilling oil seriously in 1909 and could still have spent some limited time with his family before that, and it's possible he had a very young brother named Byron, maybe 3 or 4 years old. Also Daniel was at the height of his riches in 1927 when the film ends, but only appears to be in his fifties and could live another couple of decades. He could have abandoned his family in 1908 or so, and his much younger brother might only remember that he was "an asshole". And it's exactly in Daniel's character to leave a measly fraction of his inheritance to a family member. Only the surnames don't match. Daniel could have changed his for various reasons around being a loner.
    Hadley: So this shithead lawyer calls long distance from Texas, and he says, Byron Hadley? I say, yeah. He says, sorry to inform you, but your brother just died.
    Youngblood: Damn, Byron. Sorry to hear that.
    Hadley: I ain't. He was an asshole. Run off years ago, family ain't heard of him since. Figured him for dead anyway. So this lawyer prick says, your brother died a rich man. Oil wells and shit, close to a million bucks. Jesus, it's frigging incredible how lucky some assholes can get.
    Youngblood: A million bucks? Jeez-Louise! You get any of that?
    Hadley: Thirty five thousand. That's what he left me.

Daniel did really care for H.W.
The reason why he hated that H.W. went into the oil business was because H.W. was on the steps to becoming like him. Since Daniel deep down hates himself he starts to truely hate H.W. for becoming a reflection of himself.

Daniel didn't lie about paying $10,000 to Paul
It is entirely possible that Daniel just gave him more money later for more locations or, if we take Daniel's words literally, for actually warning Daniel of Eli's greatly manipulative tendencies. It is also entirely possible that Daniel did keep tabs with Paul after the fact -since they didn't part in exactly the worst of terms- and that's how he found out he was doing well enough for himself.
  • It might be possible that Paul invested the $500 profitably and ended up making much more money, and then Plainview folded that later information into his insults to heighten the blow.