You don't have to choose anyone you feel is incapable of raising your child the way you'd want. But a godmother's responsibilities are what you make them. Most likely, the person who'd get custody in the event something happened to you is your baby's father. If he passed away, the person you designate in your will as guardian would get custody. (It might be a good time to draw up a will, if you haven't already.) So don't focus on what she won't be to your child; let her know that she'll be a much-loved person in your child's life.
Q. My best friend wants to be godmother to my firstborn, but she's very irresponsible. How do I pick someone else without hurting her feelings?