I hope you managed to keep it together for boxing day - you sound like the kind of person that would, OP! No point in allowing your sister's appallingly immature and self centred behaviour to ruin yet another family gathering for you and yours, and extended family.
When I make a commitment to look after other peoples' children, I'm making a commitment to look after them until the time that I have said they need to pick them up by - family or otherwise. Any time after that, and they're taking the piss, and there's no excuses. I say that as a person who didn't have a night away from my kids until they were 7 and 9. I didn't have the privilege of family close by who could or would take them for the night. My choice to have kids, my responsibility to look after them. I've had someone do something very similar to me with an older child, leaving that child with me well past the time agreed for collection, and the child was confused and worried - and embarrassed, because it had happened before, and sometimes people are a little overt in their frustration and anger at being put in that situation. (I tried not to be - it wasn't the child's fault).
I really hope, OP, that you very, very, quietly, (that's always even scarier than shouting), make it very clear to your sister just how much her selfishness disrupted your families very carefully made, carefully earned and paid for, and much look forward to very special plans. I hope that you make it very clear to her that her short-sightedness (and cowardice) has cost her a very valuable resource, in terms of support and childcare in the future, because 'fool me once, shame on you, fool me twice, shame on me'.
And if she did have the temerity to approach you for childcare for the wedding in Ireland, I hope you'd just look at her, and laugh, and say, 'you're kidding right? Hang on, you're NOT kidding?! OMG, you might have that bloody short a memory sis', but I sure as hell don't - not going to happen, not now, not ever. I did tell you, remember? Actions, consequences m'dear. You need to learn that, so you can teach your kids a little bit earlier in life than you seem to have learned it!'