There is a way to do this the non-lethal way really quickly. Unfortunately you don't get much opportunity to steal things from the upper floor since you don't even go there.
Talk to the guy in the rat mask and he will identify the Boyle you need, then talk to lade White and she will ask for a drink. Get her one and then she'll tell you which Boyle is which. After that you just get her downstairs. A bit of a boring one but it's less risky than going upstairs and might be a better path if you are bad at sneaking etc.