In actual fact, she was smart enough to figure out exactly how much effort she needed to put in so that she didn't fail and was able to coast through school with minimal effort.
Ironically, that was also me, during most of my middle/high school years. I knew the exact amount of effort it took to leave me at exactly around A-/B+... smart enough to never be in trouble, but not so good I attracted attention. Smart enough that my parents were "moderately proud" of my aptitude and left me alone.
There was a 2 1/2 year gap between my finishing high school and going into college. On entering college I CLEP'ed out of essentially all my freshman and sophomore classes, and about half my junior year equivalent... and then screwed around for five years, basically taking classes I thought would be useful/interesting... completely independently of any degree plan. When you start college with 78 credit hours with A's, you can pretty much do what you want.
Then I graduated and have been various versions of self-employed for 30+ years-- never used the degree for anything.
Point here being... I always did my best when I was the only one I needed to impress; I cared less when it was someone else's expectations of me that were on the line.
Selfish? Perhaps.
If your sister is content with her life, bully for her! The thing with my brother-in-law is that even though he "accomplished" what he was trying to accomplish... it didn't make him happy, it just made him invisible.
Could it be that invisible is exactly what he wants? Some people like to sabotage themselves so that they can never be happy, but rather simply exist in their own world.