As for the night he staggered out to the sitting room, it is possible as you suggest that he did it in a fit of drunken superstition.

But there wasn't any money on his conscience.

Couldn't be for there wasn't any.

If he feared at all to have his sister revisit her home--queer notion, that, Mr.O'Neill! You Irish run to notions!--it was simply because he hadn't given her kids a square deal and he knew it." Again the doctor adjusted his glasses and went back to the will.
"Doctor," flung out Kenny desperately, "I myself have seen indisputable proof in that house that Adam Craig was a miser--even the way he handled money." The doctor sighed and looked up.

And he smiled his weary, understanding smile.
"What you saw, Mr.O'Neill," he said soberly, "was something very close to poverty.