You HAVE to stop being so nice for your own sanity girl! Your sister sounds like a needy, more, more, more, greedy little cow and if she doesn't want to participate in your fantastic holiday atmosphere then tell her to go and fek herself.
There is only so much you can do with people like this and you HAVE to draw the line! She is taking the piss out of you and your warm hearted, generous and level nature.
I tell you this...I bet that she would still want more if there were nothing left to have or give and people like this are poison. You would do best to distance yourself and hopefully teach her a lesson. Make it the BEST ever without her and when she hears about how good it was she might eat a little humble pie.
GET HARD. Tell her that the kids are welcome, but you wouldn't want her anywhere near you at such a lovely time of the year. She just does not appreciate you.
I would be DELIGHTED to come over and share the holidays with ya!