Okay, so I have a sapient alien species that has a carbon-based biochemistry, and is extremophilic, able to survive having their internal temperature lowered to as cold as 253 K. They are able to do so by having a substance dissolved in their blood and other bodily fluids that lowers the freezing point of water by 20 K, as well as using proteins that can function at low temperatures. What substance (or substances) can lower the freezing point of water by that much, and is nontoxic enough for an organism to survive having it in their bodies 24/7? And, can be produced via biochemical reactions?
The easiest thing would be to use ethanol... it mixes nicely with water, and you can choose a sufficiently alcoholic mix to fit your cold-weather needs. 40% ethanol by volume is enough to keep a pure water/ethanol mix liquid at -20°C, and I'm sure it can work for your needs, too.
Ethanol is readily synthesisable from a whole range of carbohydrate feedstocks and is a useful solvent in its own right. A mixed ethanol-water biochemistry is inevitably going to be a fair bit different from our own, but not so wildly different that it won't inevitably involve carbohydrates, lipids and proteins. One thing you can be reasonably assured of is that they won't get drunk on the same things we do... the fact that high levels of ethanol are toxic for the biochemistry of most terrestrial organisms doesn't make it intrinsically bad to all possible forms of life, so it would be necessarily harmless to your aliens.
They may not like hot weather, due to alcohol's lower boiling point and higher vapour pressure than water at the same temperature. Alcoholic sweat will help cooling, of course.
Cremation is likely to be a risky end-of-life choice for this species. In fact, their general relationship with fire seems likely to be a much more cautious one than our own.
Antifreeze proteins plus urea
Most real animals use antifreeze proteins. they have independently evolved multiple times.
by itself this is not enough however, the limit on known antifreeze proteins is -13 degrees C. However another common method is retaining Urea in the blood, many deep water fish do this. so try combining these methods.