if i was you, i would apply for a visa for her, it shows more commitment from you, that you do intend to marry if you bring her daughter. strengthens your g/f and her daughters visa app. surely your g/f and you would want her daughter/your future stepdaughter to be at your wedding and in the uk with you
how old is your g/fs daughter ?
if you leave her behind, then you will have to provide evidence when you do apply for a visa for her, showing financial support, also who's been looking after her etc.
is the girls father mentioned on her birth cert? does she have contact with her father? you know your g/f has to prove she has 'sole responsibility' for her daughter.
a good letter of support for your future wife, and mentioning your future stepdaughter, stating her future is as important to you as your futures wife's is. and leaving her behind is not a option as your family would not be complete if separated should help
good luck
