Now maybe I'm biased because my degree (hey, my diploma just arrived last week, so now it's "officially" over!) is in a field that, well, really doesn't help me do anything. And my year of college before I transferred is in a field that really really doesn't help me (animation.) But I'd like to think that just because I don't work in either of the fields I studied in, my school wasn't a waste. In fact, comparing my skills and abilities before college to what they are now, I know it wasn't.
Now, if your sister is just making excuses, then bah on her.