I don't see why everyone's making out the sister is being an arse it was obviously a tongue in cheek comment after you've sent her some pictures of something that looks like a brand new, pristine home. It's not "ha ha! I shall release my feral children, good luck!“
Ignore the bashing furniture, she doesn't mean that specifically, she's acknowleding how nothing looks out of place and made a bit of a joke by saying, can they even come in? You got instantly offended and precious over your house, and that's probably why she then made a dig about trying to keep the kids on a mat.
My house is white, pale grey, blush pink and gold. I know my mates shudder when they come round with DC, and the first time someone comes round, they'll often make a funny comment like "omg just stand on the doormat and don't touch anything". And I laugh, and tell them to go play with DS.
I've had loads of kids round at Christmas. Sometimes things get broken, spilt, bumped sometimes they don't. Sometimes it's totally accidental, sometimes it's down to them being over excited and a little careless, It's not a reflection on behaviour, or the parents, it's what happens with 2 and 4 year olds, especially in Christmas day.
Yanbu to be proud of all your new stuff. Yabu to invite a 2 and 4 year old when you can't accept they might knock a drink over, or drop a toy on the coffee table and leave a mark. If you're that precious over it that you can't enjoy your day for fear that something gets accidentally scratched, then it's your call. You need to make that decision.