Corn snakes are great and all, but the choice is clear. Ball python for sure.

Corn snakes are for people who like colubrids, coloring, and ease of care. I've had ball pythons and corn snakes, and would always choose a bp over a corn. Corns are just to....hectic for me. I like a kind of snake who will sit with you on the couch and relax, without your spending your time keeping it under control. I also have a tendency to boids. I like thick bodied snakes. I remember way back when, I wanted my first bp and kept hitting a wall with the parents. They thought I'd lose interest, all kind of things. But the answer I got every time was "You're not getting some damn muscle snake", yeah my dad calls them that to this day. I also prefer a ball pythons pattern over a corns, just my preference, looks exotic. Corns are easy to care for, they barely require anything. And for me this turned out to be a bad thing. My first corn was too easy to keep my young mind interested, so I kinda put it aside. Plus, it's fun caring for bp's special needs.

I'd check with your sis as to which kind of snake she'll want. Just say something along the lines of "I'm buying you a snake for christmas, what do you want a ball python or corn snake." That'd be so blunt she'd never know you were buying her one

This is a site full of ball python people, I'm sure you'll get the same type of answer.

*Not bashing corns! I've kept two, great snakes*