It is being suggested that as well as having looked after her stepchildren because their mother made a mess of childcare arrangements, OP should also be punished by having to cut short her holiday with her own children in order to suit the convenience of this woman who is apparently unable to arrange childcare during the days she has her children. So yes, the OP clearly ought to be punished for being a stepmother. – Evil, evil, shades of Snow White and Cinderella; you'd think women in the twenty-first century might have grown out of children's stories from the eighteenth and nineteenth centuries, but apparently not.
Oh, and be castigated all over this thread for the crime of, well, having had children, and existing, as far as I can make out. Burn the witch! They'll be saying she ought to be made to dance herself to death wearing red-hot iron shoes, any time now.
(No, I am not a stepmother.)
Whilst the man who is one of the originators of the situation has been mentioned about three times except as having, with his partner (that's the cuckolding ex), taken two children to Spain on holiday in May. And only by me and one other person (maybe two?) as perhaps, just perhaps, being on the spot and possibly, just possibly, able to look after his two stepchildren for a couple of days.
So I thought I would remind the righteous of his existence and culpability.
And I don't think the children's mother is unable to look after them; I think she is choosing not to want to, which is a slightly different matter. Supposing her ex were working on an oil-rig, she would be unable to summon him home to fill in for her deficiencies; but she feels fine about pestering him while he is on holiday, because how dare he look after his other children during the time she has the ones that are hers? There is no emergency, just poor planning on the mother's part.