The prevailing attitude to the virus on these forums (that generally, restrictions have not been strong enough) is way out of sync with what I see in many places elsewhere (online and in real life). This doesn't just apply to Christmas; it's applied right the way through.
That doesn't say anything about what's "right", it's just a reality. We can sit here and talk about tragedy of the commons, but to some extent we just have to accept that a lot of people don't agree. They might believe the risk is over-stated, or they might simply be prepared to take the risk, do stuff and know that a bunch of people will die anyway.
Criticising someone for considering a risk-managed visit to their family at Christmas has to be in that context, especially if there's no official guidance that specifically says not to. If there's no guidance that says not to, then the fact is, a load of people are going to go and mix at christmas, and yes a load of people will be ill/die as a result. That'll happen whether or not someone with a relatively cautious attitude does a family visit where there is indeed some risk but in relative terms it is small.