Fairly certain it is the son who betrays you as I remember the assassin saying that Lord Grathan(aka the son pretending to be Lord Grathan) was the one who ordered the kill. Hmmm or maybe it is just the son if you.....have intimate negotiations with the wife.
Remember the wife speaks through the son as she said in her plan. Plus the son does not seem smart enough to do that and it seems that he listens to what ever his mother says. Lets not forget this happens to both male and female sith warriors that choose to work with grathan's wife.