Daniel Faraday is Charles Widmore and Eloise Hawking's son

As Eloise Hawking is leaving the Marina Medical Center in Long Beach, Charles Widmore is waiting for her outside. Widmore asks her if Desmond is alright and she says that he's fine. Eloise tells him that Penny is inside and that he should go inside and say hello. Widmore says that his relationship with is daughter was one of the things he had to sacrifice. Eloise gets angry and says "Don' you talk to me about sacrifice Charles. I had to send my son back to the island knowing full well that..." Widmore interrupts and says "He was my son, too, Eloise." Eloise slaps him across the face then turns around and gets in a cab.

So it was a little surprising, but not monumental that Daniel is Charles Widmore's son. It was not as obvoius as the Pierre Chang and Miles relationship. Charles widmore uses the word "was" when referring to Daniel being his son, so he seems to know that he died in 1977.