That’s wonderful actually. But I’m talking about your infinite amount of excuses..” Liz is responding to her little brother very sarcastically in the first sentence and ending this line pretty sternly like chastising.

“Oh my goodness…” She’s saying this line as if she’s rolling her eyes. Responding to an asinine or silly statement.

“You’re a knuckle head now, but you’re MY brother, ok? You’ll make a difference! I believe it!” Pretty self explanatory I think. She’s pretty positive and very upbeat in this line.