Your wife is with me now. She says fuck off you selfish bastard. Grieve for her forever and then some more and don't even think about shagging someone else in your marital bed.

However, she admits that a little light flirting with her sister could be flattering but only if you squint and pretend it's her. Dressing Sandy in your dead wife's dressing gown might facilitate this so I'd say, go for it.