Its a very very long and increasingly boring even to me story so i won't go into it here. Suffice to say that my sister is so competitive that she even wants to win some sort of popularity contest with me over my childrens' affections. I'm managing to keep her at bay most of the time but she also spends nearly every day with my mum as my mum does all of her childcare and lives about a mile away. 
Another long story.
So when i invited my mum and dad along to my dd's first Xmas concert, mum piped up that sister is thinking of coming instead of dad. I was really caught out here and just went 'oh' and didn't have a come back. So upshot is i now really don't want her to come but need a reasonably believable excuse. I don't want to be up front and say 'you're not invited' as it just feeds her victim complex causes more problems than it solves. But if there was a reason i could use, it would keep her off her high horse and out of my hair keep the peace. Any suggestions?