I could see Sophie as the accomplice with her being the "artistic" half of the duo: SBK kills the victim since he's stronger and/or a professional; he then signals to Sophie to come in and then Sophie frames the tableau.
Would fit her being an artist and also explain why she is more accepting of Jack; she sees herself in him.
Problem with this is if SBK is a professional, how does Sophie pay him? I could see her being "estranged art-school daughter of wealthy Bostonian" as a way to pay a hitman, but I don't recall any discussion of her family life.
Another option would be SBK is her father/family member, but then that would mean that Maddie/Mitchell/Tessa never saw a picture of Sophie's family since she was with Cam. (Garrett wouldn't have been around for the past 14 years, so that at least makes some sort of sense).