It may be a famous movie. I saw it some 10 years back in TV. A family moves to a house which is haunted. I think there is a 15 year old girl and a little boy. There is a man who works in the farm, and there appears a murder of crows associated with the presence of a ghost.The ghosts are really evil. At the end, the man in the farm has something to do with the ghosts in the house. Can anyone tell which was it?
Could be "The Messengers (2007)".
Unemployed and running short of money, Roy moves his family to a deserted farm house in the hopes of cultivating sunflowers and selling the seeds for profit.
Once at the house, they witness a series of strange events and soon enough we learn of the tragic incident that took place in the house years prior.
From this page, some of the details you mentioned:
The house always has crows flying over it. Some attack Roy [the dad]. But they're driven away by a man named John Burwell, who Roy hires to help with the farm.
And from the movie's Wikipedia entry:
While at a local store, Jess [the teenage daughter] sees a newspaper clipping of the family, with the father revealed to be none other than John Burwell, who has always been kind to her.
Spoiler about the ending:
As it turns out, John is actually John Rollins, the man who, in a fit of madness, murdered his entire family, shown in the beginning of the film.
There was a prequel movie released about the story of John Rollins named Messengers 2: The Scarecrow which is truer to plot originally suggested for The Messengers.