I have an almost 6 year old who is exactly the same - he even eats his cereal by lifting it out his bowl wuth his fingers and putting it on the spoon. He has been known to eat soup with his fingers as well. I don't know what the solution is unfortunately. I have tried to get him to clean up himself afterwards, but it doesn't make a difference. He is not too bad when we take him out to eat, so he obviously knows the difference. He can keep himself clean if he is wearing his favourite Brazil football strip!
how is your step daughters co-ordination in general? Is she normally clumsy? It could be that her fine motor skills are poor rather than that she is being deliberately careless.
I know that my son is careless as he is not normaly clumsy.
I presume that you have tried reward charts etc. What about buying her a top that she really wants and tell her that she must keep it clean or you will throw it away