Queenie accidentally gave Arthur Knockout Drops in his coffee.

Evidence: It looks like a morning scene because there are eggs frying. 50.8 percent of men consume coffee. Most women make breakfast. She would only make breakfast if they had a relationship of some kind. If they were related then there is a reduced likelihood that she killed him. Arthur would most likely not lay asleep at the bottom of stairs.

Semi Ballad:

Queenie wast making coffee one day,

When “Where is the milk” she did say.

She scrabbled with one hand and no eyes in a cupboard.

There was nothing but knockout drops, like old mother Hubbard,

Without looking at hands she did pour liquid in.

When Arthur did drink it ‘twas accidental sin.