Choosing God-Parents

So I need some serious help with this matter. As far as the God-father, I believe it's already picked out but everything is so up in the air. So I've had this dilemma with choosing the God-mother because my bf wants to put his sister but he's already chosen the God-father so I feel like that's unfair. Now she is a doll and has been such a great help, and always asks for the baby so that's why I've considered allowing it, but I just feel that she already has such an important role as the aunt. I'm torn. What have you parents,that have or will baptize your kid, take into consideration when choosing your kids' God-Parents?