Why about the possibility of brother instead of brother-in-law? My reason being the stronger the bond, the greater the betrayal, and consequently, the greater the dramatic tension when the hit man discovers he's been conned (by the one person he thought he could trust).
And how is he "forced"? IOW: what is ultimately at stake that compels him to turn on his mentor? Seems to me, it's got to be more than just the money.
Whatever, I can see how the premise would put the hit man not only in jeopardy but in the horns of an interesting moral dilemma.
Best wishes with the premise.