She sounds so sweet that sometimes you just don't see it coming.
A few examples.
Player: Does it bother you that we're working with your father?
Serana: I always thought it would be more of a matter to you.
Player: Why would it matter more to me?
Serana: Wait, so you are that naive? Oh. Wow. (Proceeds to explain that Harkon can't be trusted. And yup, she did say wow.)
_____
Serana: How about you?
Player: We are very close. I can't wait to see them again.
Serana: You're a good son.
Player: I try.
Serana: Yeah. (Read it as a drawn out "Yeeaahh..." with a sarcastic tone)