I'm a step mum myself, so this is not step mum bashing.
I live a way from my sons Dad, and visits are only around every 5 weeks. The night before the last one, his SM phoned my son and cancelled it, saying that they had a social engagement that my son would find boring. I was really taken aback. He's 11 years old. I had to cancel a trip away myself that weekend, and had a disappointed son on my hands.
I got my son to phone his Dad, and they said they'd rearrange, but they haven't.
P.s. I've got a lot of stick about that from his Dad, as he says he'd like to see him more often - I'm the one who pays all travel costs despite being the main parent - and he's welcome to pay some travel himself but doesn't.