I think it was Anthony Coleone on the advice from his fav. uncle Fredo to leave the drapes open. Then he drew that painting to distract his father for the shooting. Motive? well he knew after his father he was the smartest Corleone left and we know he wanted to help Michael when he asked to go with him in the scene where the two are talking in the bedroom. This is totally for a laugh but it does make a small case.