Oh dear...yes I can see this is potentially hazardous at Christmas time! I went thru a similar phase with DD1 re loungeroom dancing, I just calmly explained that I was enjoying what I was doing and would like to continue with it and she could join if she liked or she could go and play in her room if she didn't like it. Not quite the same as singing though as I imagine sometimes you can get "noise" overload (you know if you're a bit tired or stressed, the level of the noise can be overwhelming - some ppl are more sensitive to that than others). Do you think this might be the case - or does she just not fancy it right at that moment?

I suspect in a group situation (like party) if you tell her they are going to keep singing because they like it and everyone else likes it and give her a choice of staying to listen or leaving, I bet she'll choose staying to listen. In the car though, not sure about that as she can't choose to leave - maybe that's an area where you will have to compromise for her.