The correct answer is Buck Anderson.
"Didn't there used to be six cards?" Amy asked.
Rhoda crossed the room and inspected the mantle. "You're right. A photo Christmas card from Aunt Karen and Cousin Tim. I wonder who removed it."
Amy answered. "Someone who didn't want people looking too closely at it."
Amy turned to face the others. "Mr. Weaver was hit on the head and killed. The stepladder was then laid on its side, a bit too perfectly. The killer went upstairs, to the room above this one, placed a stick down the register and knocked over the tree -- to attract our attention."
Buck shook his head. "The bedroom above here is mine. Are you saying I killed him? Why, for heaven's sake?"
"Your real name is Tim, isn't it? You're the missing nephew. When Uncle Edgar got in touch with your family in Florida, you saw your chance. But instead of coming here as yourself, you put on a fake mustache and wig. No one would suspect a stranger without a motive. You could then go back home and cash your inheritance without anyone ever meeting you again.
"The only thing that could give you away was the card your uncle had so proudly displayed. No one had recognized you yet, but someone might. It took just a second to tear it from the mantle and throw it in the fire."