How do i tell my dad that i dont like his fiance!?

My dad is getting married to a 19 year old girl and in wich my dad is 39!!!! she used to be my falcon buddy! :( ewww! But i want to know how do i tell my dad that i dont like his fiance! shes a neat freak, treats me like crap, and is a canniving *****! HELP ME!!!!!!!!