Ditto what everyone else has said. You do what you and DD want. If people can come, great - if not, great. You'll see them on Christmas Day anyway. And can she have a party with school friends on her actual birthday? Sorry but SIL is out of line - this is not her decision. This problem will never go away so time to do what suits you and your little family.

ETA Sorry I posted while you were posting! Sounds like some great ideas you have happening. And as someone who has done their Christmas shopping, no you're not thinking too far ahead! Don't forget you'll have the benefit of daylight saving so you could even do a lovely twilight party, and if there are Christmas lights nearby you could go look at them. Even a pool party at that time of year with a bbq would be great. I'm sure it will be awesome!