Judging from the Aya's summary, has to be the butler, right?
He's from the mansion, he intimately knows the mansion and the residents, he can touch the spike (which excludes the eldest son), he doesn't have an alibi (the maid was with the daughter, the second son was with the father), he'd be able to sneak up because the victim trusts him.
Seems to be the obvious answer, wonder if it'll really be that easy. Or maybe the maid did it before spending time with the daughter?
I could also see it being a suicide as well. Remember, she was originally human and now has or had a much longer lifespan.
This would also explain why it didn’t pierce the seat along with the peaceful expression. As for the stake moving, the son who went hunting could’ve moved it to the storehouse after the fact but didn’t tell his father.
He hurried his son back home because vampires decompose quickly.
He bathed the stake in animal blood and then a large amount of his own blood. His blood evaporates, the animal blood does not, thus faking a fresh kill.
The little girl was tricked into thinking the victim is not dead.