Personally I think the government should have banned all household mixing over the xmas period
Now some people would have ignored this instruction. However don't forget ....
1) Lockdowns have mostly been very well observed. Obviously some regions and ages have responded better than others...
Well, it's that some still haven't (incredibly) grasped the seriousness of the situation or think that they and family are immune from the risks and will make up a story which they tell themselves and other people as to why it is safe and sensible to do so