Actually, if it was me, I would stay out of it. No wait, it was me..................lol......oldest son and oldest daughter, she worked for him. I told son, owner of the business, that at work, she was his employee and to treat her as such, that way, less resentments from other employees.
I told daughter, that the 'blood thicker than water' bit stopped at the door of the business, she was just an employee and had to earn the respect of the owner (her brother) and the employees, she was in the business world now.
Kept me out of the middle of it (it being anything workwise, lol) and let them both know I loved them dearly.
Just what worked for me.
Love and hugs,