with a pen

a marsupial is an animal with a pouch to carry their young

Barrymore

Unfortunately she can, assuming she is the biological mother and you are a stepdad who never adopted the child and gave her your name. If this girl already has a father in her life there would have been no reason for you to adopt her. Sorry, if this is the case you don't have a leg to stand on (legally). But, if the child is old enough to tell her mom that she wants you to stay in her life, and the mom is willing to put the daughter's best interest at heart (I have to assume you have never abused her in any way), maybe a simple conversation will be all you need. Good Luck!