Using a space heater isn't going to do much if the entire situation isn't optimized to keep everyone inside warm.
The absolute No#1 thing anyone can do to get warm in a very cold environment is to pitch a tent inside the warmest room in the entire house ( usually facing south) Layer the tent with blankets over the top, then fill the inside with more blankets/towels/clothes to form a kind of nest. Then sleep in a sleeping bag inside, while wearing a beanie, mittens and socks and layered clothing. Use a hot water bottle. Also while drinking hot liquids and "grazing" on something like celery.
The act of constantly eating something small and light will help keep you warm. But the bigger issue is to trap air.
This is where plastic sheeting rolls, big ones, and duct tape are useful. In a two story situation, move everyone into the lower floors, and use plastic sheeting to seal off other areas to the house, particularly higher ceiling points.
A lot of people want to be warm but don't like "inconvenience"
I've been homeless before. You catch a very bad case of the flu and you have to live outside full time, you can die out there. If the weather gets very cold, you can die out there. If you cannot regulate your core temperature for long enough, you won't make it.
I don't see a problem with a space heater, but it should be IMHO lower on the list of ways to optimize keeping a place warm.
I have a ten year rule for a fall back plan. Lots of people believe they are "in a good place financially" OK, if you cut off all your income streams, how long could you last? 3 months? 6 months? A year? If you can make it ten years straight, with nothing coming in, and meet your basic needs, then I'd say you are in a good place financially for the short term. For people who are not, squeeze every penny you can. If I had a choice between turning on the heat or just putting on a beanie and a sweatshirt, when I was middle aged, I'd put on the extra clothing. Cut the cable, cut your gym memberships, cut your Netflix, cut your weekly food box delivery subscriptions, cut your Amazon Prime, cut all that out. If you don't have a ten year reserve for bare basic necessities, I'd even cut out eating out.
I'm not here picking on the OP, but in general, most people have to look at doing way more with way less.
I am not cheap with other people. Many times and in many ways, I understand the importance to sometimes be generous, thoughtful, kind and measured with others. But I am mostly frugal with myself in many areas. I see prosperous times as an opportunity to prepare for the hard times to come. I did not know the pandemic was coming, but I was not spread out too thin, and I had a warchest in place, so I could weather that storm with my businesses.
Another reason to live lean is it keeps you hungry. It reminds you everything is a fight, everything is a competition and if you fail , you starve and you die.
Those were my options. Figure it out and find a way to make it happen. Or end up back on the street and die.
Do not think like a poor person. But do not think like a rich person. Think like an adaptive person.
I'm not judging the OP and I don't know the specifics of the situation,but look at the inlaws. They are beholden to someone else's household and it's very likely they don't have control over their own destiny. In so much as that's possible. Hence they have to suffer through the cold.
There's a lesson here - Put your destiny in your own two hands. If you have to bleed for that, so be it. Better to bleed yourself than to be bled by others. Because I assure you, the blood is going to get taken one way or another. This is the nature of life. This is the nature of how this game is played.
