What do you think about me telling my dad's wife her family's not my family?
Why are you going to tell her that?
My dad's wife will treat me like her son instead of her husband's.
That makes sense to be upset about. It's good that you're standing up for yourself and letting her know that it makes you uncomfortable.
I'm going to tell her that she needs to treat me like my dad's son and not hers since I hardly know her even though she married my father.
Being assertive and expressing yourself is a great idea. It will let people know exactly where you stand concerning the matter.