It’s not illegal for a prosecutor to make a deal. Happens all the time. Cohen was facing 70 years. Will do three. His wife not even charged.
Flynn’s son was facing prosecution. So he made a deal as well.
It’s dirty as hell to drag family members into a prosecution. The law doesn’t provide for that.
It's illegal for a prosecutor to make a deal for an illegal purpose. Suborning perjury is illegal.