I am not one of the legal bods on here but I am assuming that if you go through an adoption process and legally adopt the child as your own then you would have legal custody should anything happen to her. I am thinking that would be the easiest way but as I said, I am not one of the legal bods and they may have a better solution for you.
I knew that used to be the case murraymints, but I wasn't sure if you could still do it.
I was once told Iwas being"left" 2 children in a friends will (about 20 years ago). Thank goodness they are grown up now - they were bloody horrible kids