This is a formula piece. It's been done a hundred times. The greed of the relatives causes them to try to get the old lady committed. Their motives are so obvious that any chance they had goes down the tube within minutes. Now they're in over their heads and have to use their resources to survive. Meanwhile, the red herrings start swimming upstream. I should have hated it, but the crazy overacting and general being of the film are rather pleasant in a maniacal sort of way. The conclusion is satisfying and justice is done. The one drawback is that the bad guys are so pathetic and incompetent that they don't pose much of a threat.
Review of Murder by Invitation
