Walker and Radcliffe are the same person, his name was Walker but he went by Radcliffe professionally as an artist.
Isabelle was his wife, his pet name for her was petal.
The plot as I understood it was Radcliffe practiced alchemy and the occult, they made some kind of sacrificial pact and after killing her in some kind of ritual he then killed himself. That was in 1845.
In modern day the family arrive hired by an unknown 'American banker' who hired the mother to restore the house after a 150 yr old will surfaced and he inherited the place, the only stipulation being that he needed to restore the house to it's original state to get the deeds.
Some ghostly occurrences seem to the mother to find a hidden room that was not on the plans to the house. While breaking into the room the husband cuts his finger and that begins his progressive possession by Radcliffe.
He makes a potion that will allow Isabelle to posess the mother and puts it in her water bottle, she only takes a sip but apparently that is enough.
As the clock approached midnight the husband, who is now fully possessed begins to repeats the ritual that radcliffe and his wife performed in 1845, which will transfer Isabelle's spirit into the mothers body.
It seams as though the children stopped the ritual just in time and as it was not completed by midnight the spell is broken and everything goes back to normal, the husband is no longer possessed and they pack their bags and leave.
However, in the taxi it is revealed that the mother and father and both now possessed by Radcliffe and Isabelle.
I liked the plot, it had potential, the film needed much better writers though, there were too many unnecessary cliches which made some parts not make sense.
Like when the children were stuck in the root cellar who was running around on all fours?