My opinions:
One can be addicted or obsessed with anything--including drugs, alcohol, food, money, another person, sex, etc. I think the Devil can describe that type of personality--like, "if one is good, two is better..." or someone always on the lookout for the next cool thing to come along to "seduce" her.
It can also describe the basic animal nature of humans--because, after all, we are animals first, and we need certain things to survive--including food, sex, etc. But sometimes those lines get blurred and we overdo it in our quest to survive and live on as a species. I think the Devil addresses those kinds of issues, too, to remind us that our basic instincts will always be there, no matter how "civilized" we become.
As far as your readings about your sister that keep having the Devil show up--if the position is for something unexpected (something you don't know about her), and that's where he appears, it could mean a lot of different things that you don't know about her that may surprise you. That's why he appears in the unexpected slot--she could be obsessed with something you have no idea about (and that she has successfully kept under wraps from you.) We all have our secrets, I guess. Maybe she is just in closer tune with her basic animal instincts than you think she is.
On the other side of the coin, the Devil showing up in one of those positions may be warning
you not to become obsessed with knowing whatever her secrets are...that you already know what you are entitled to know. Or something to that effect.
