Think of how hurt the oldest will feel if she comes back from her Dad's, having done nothing 'fun' whatsoever - and her little sisters are all "well, we went to a party and all the family was there, and we had so much fun!"
That's the start of the resentment wedge, because she's already been told that she's not the cousin of her sister's Aunt/Uncle's baby - her withdrawing after that nasty little aside? Was because she was hurt. Beginning to realise that these people whom she loves as family, don't feel the same way about her. That "othering" was deliberate and spiteful, designed to put a child back into their box labelled with "not a part of our family".
As a mother, I'd be drawing diagrams about immediate family (@ThoroughlyDisgruntled, her husband, their 3 daughters) and then extended family - everyone who thinks they ought to rank over a little girl and her relationship with her step-father and her sisters. Sure, she has a Dad and possibly siblings from him, too... but her primary family is the Disgruntled one.
If it were me, I'd vote with my feet... and if my husband couldn't see why, I'd ask him how he'd feel if the two of us split up, and my next husband/his family treated his daughters the same way that he/they are treating the oldest. Because I can guarantee that he'd hate it, on all counts. But I wouldn't be shy in calling them out over their exclusionary tactics, either. Willing to bet now you're aware, you'll hear a lot more of them, dressed up in naice words and tone of voice... but there even so.
Your daughters are sisters in reality, half-sisters in biology only. Family's supposed to be about more than just biology, surely? Otherwise, why bother?