Poor old you. Feeling exhausted and down is all part of having a very bad cold. See the doc to discuss the bronchitis fear - very sensible - and maybe also mention your state of mind. The totally medically unqualified Mosaic barefoot doctor advice is this: do what makes you feel less under pressure. So if the thought of cooking for 10 is upsetting, how does it feel if you give yourself permission NOT to cook for 10?
Try it now in your head. Makes something lift? Then there's a bit of an answer.
You're family sound like there's at least one other grown-up - contact these folks and explain that you will need them to muck in and sort it all out as you aren't so well. Or perhaps even move the whole shebang to one of their homes, so you don't even have to play hostess.
I bet by the age of 30 you, like me, were running the whole show more or less single handed. Enough is as good as.
Think of it as just one meal. Sunday lunch. You can arrange that meal again another day when you feel better.
In Spain there's Three Kings Day on 6th Jan so you could plan to celebrate then instead, if you feel you'd still like to do the big hostess thing.
Think about yourself, your health, and your feelings. These are very important. Give yourself permission to be happy in your decisions. And get well soon!