It's not possible to know what's gone on without being there (and even then, there are usually two sides to a story). I agree that the arrangements should have been worked out between the parents.
The stepmum doesn't have a duty to love the child, but she does, IMO, have a duty of care towards her. If she has other arrangements for the time the parents have arranged for the child to visit, I think the dad should step up, but I also think that the stepmum should make the girl welcome and not let her know that there have been any issues.
For all we know, the mum may have deliberately chosen a time that she knew would be inconvenient, the dad may have ducked out of previous arrangements - there are numerous possibilities - but the little girl should really be everyone's priority, and should not be allowed to feel like she is a pawn in whatever games are being played.