I don't think Wendy is as smart as the show is trying to portray her. She doesn't need to kill her own brother. All she has to do is drive Ben back to Darlene's compound and leave him there.
I doubt Helen's hit man is able to go through Darlene to get to Ben, even if he did, he would have to kill Darlene first, so for Wendy, she get rid of at least one enemy.
Or, Darlene will take out Helen and her hit man during the conflict, in that case, Wendy wins even bigger, she can lose one of her biggest enemy and save her brother at the same time, all while keeping her own hands very clean.