Generally around five days, but yeah, they probably didn't get it at that family party. They may well have given it to their family members, of course (you're infectious before symptoms show).

Last I read, the median incubation periods is estimated to be some 5 days.

Here's the paper I remember looking at: https://www.acpjournals.org/doi/10.7326/M20-0504

Even now we don't really know with certainty. It's certainly longer than one day. But as far as that quote: I take it to mean "We were contagious and hosted family, oops" and not "We caught it from them".

Up to 14 days incubation, but if I recall correctly the average is five days.