A negative test a week ago means nothing here and now today! It means she was negative at the time of testing, not still negative now!
And after mixing with 2 other families at Christmas (which she was fully entitled to do) she could still have been brewing it from them when she had the test. It could now be rampaging through her system but she’s just asymptomatic. Lots of ‘what ifs’ and unlikely scenarios there but still possible
*brewing it from THEN not THEM. Although kinda still makes sense just sounds meaner