Usual advice on here is for stepmams to butt out and leave parenting decisions to their parents. I've seen this advice given to women where the step children are at their parents home much much more than the very low contact that is happening here.
If she wants advice on how to tackle bad behaviour - butt out
Advice on how to include herself in contact more - butt out they need alone time with Dad.
Yet when it comes to money suddenly everything is equal, she should be ensuring their Dad has better contact despite him not giving a shiney shit before hand.
I agree it would be cruel for him to fuck if in holiday with his girlfriends family, but I don't agree the OP is a bitch or cruel. In this situation I'd wave my girlfriend and her family off on holiday and stay behind myself.
I've had shit cruel step parents and this is the exact stuff my Dad did and I'd be liar if I said it didn't hurt. My Dad died a few years ago and relationship with step mum fell apart after as she hated us and didn't even need to pretend. Contact with out Dad would be once a month if lucky despite being five mins away.
At my dads funeral his friends and family didn't even know he had two other children. That's how much involved we were in his life and it was hurtful but also humiliating to have his next door neighbour ask how I knew him. To see every condolence message not include me or my brother but his wife and her dc and my half sister.
To be honest, looking back a holiday at that age with my step mum and her family who I don't know would be horrible and would just show how little involvement I had in my Dads life.
I'm projecting, I know I am. I have nothing to do with any of my Dads family family and nothing to do with my step Mums side. I saw them so little I don't feel like they are family to me. My step mum may have hated us but at the end of the day my Dad was shit without his wives and girlfriends and didn't take much persuading to be even shorter, he chose her, my dad chose to cast his dc aside and he chose to not be bothered by it and at 40 years old.
I think OP is crazy for being involved with someone who chooses to not have much involvement in his life, and I'd judge him for not including his children more in day to life to the point that after four years they are almost strangers to his partner. He should have been trying to include them and I suspect if OP was asking for advice on becoming more involved with her boyfriends kids she'd be told to back off.
I wouldn't be involved with him but this sounds like the OP isn't what's turned him into a deadbeat, maybe he was already one.