I have had my step daughter for 13 years and now she wants to go live with her mother, which has never been i?

A girl needs her mother at that age especially. If the mother is in shape and able to be a proper mother to her, let her go.

She's a teenager. That's what teenagers often do because they feel themselves changing and think things will be better with their other parent. They want something different that they think will be better because they haven't experienced it yet- and often their time with the other parent has been limited and "fun" (like every other weekend and 2 weeks in the summer where their other parent showers them with money and attention).

Sometimes letting them go for awhile isn't a bad thing because they will get to know their "real" parent and how "real life" is in that household - often not what they are envisioning in their minds. Perhaps letting her spend summer vacation will be a start.

let her see the other side of the fence if you have been a loving mother then she will respect you more for letting her learn both sides of this issue