You might not have symptoms of Covid-19 but that doesn't necessarily mean that you've not got it; many people who contract the disease are asymptomatic. Unless "I live by myself" means that you live a totally hermit-like existence, you might have caught Covid-19 in, say, a supermarket. (Even if you have everything delivered to you, there's a very small chance that you could pick up the virus from a surface on the packaging of your deliveries).
Further, the virus doesn't only move in one direction. Even if you couldn't transmit it to other members of your family, they might transmit it to you.
Anyway, I'm unaware of any travel restrictions which mean that you can't visit your family at Christmas. (There's a legal limit on how many households can meet up together, and there's advice to avoid using public transport where possible, but there's no specific travel ban).