My guess is that he'll be forcibly shoved out of the house on Christmas day, will fall down the steps and die on the pavement - either of a head-wound or a heart attack, or both.
Afterwards there will be confusion whether he lost his footing or was pushed, and who did the pushing. If he was pushed, it was probably a joint effort! Other spoilers suggest the Brannings all getting their story straight for the police, and this would fit too.