People are really daft about the cold on MN. In Russia and cold countries, they understand how dangerous the cold is, to the young (who cannot regulate their own body temperatures) and the old, so they spend a vast amount of their time and money heating their homes. In Eastern Europe, I am never cold in winter, they have extremely effective heating delivered centrally to their flats, or if you go into the countryside, they have wood burners/oil fulled heaters, they would never ever suggest leaving a baby in an unheated house! If they have no money, and are poor, they live in one room and sleep in it altogether, for the entire winter. They don't go upstairs, simple.
And, as for all those saying, we didn't have central heating, what, you had no heating whatsoever for the entire winter? If you have even one room downstairs with a coal fire, it heats the upstairs rooms just a tiny bit. That's why I wouldn't go unless they were heating the downstairs rooms, I'd take a fan heater/oil filled heater and have it on.
But you can end up overheating a baby in a cold house by overdressing/using duvets etc, you just have to be sensible and not over-react.
I wouldn't personally go, I hate being cold and would spend the entire weekend shivering and feeling miserable, as I find once the cold sets in, it's hard to get warm again unless they are happy for you to all sit under duvets!