Isolation is isolation whatever time of year you are unlucky enough to be placed in that situation .
Isolation means that you have to stay on your own, without mixing with other people and is supposed to stop the spread of this illness.
So your husband is quite happy to not only possibly spoil his own Christmas , but yours and your family's Christmas by his absence, as well as possibly spread Covid as well.
How kind that he doesn't want to affect his own parents, but your family and GC don't matter!
Can only suggest your stepdaughter tests each day as advised and your OH takes her to her mothers, hopefully , for New Year.