Hello, Your thoughts on using flannelette fitted cot/bassinet the sheets in winter? Thank you!

Red Nose does not suggest any specific type of material for cot/bassinet sheets, but recommends that any bedding is a tight fit, so that it does not become loose creating a risk of baby’s head & face being covered,
https://rednose.org.au/article/bedding-amount-recommended-for-safe-sleep

If you are considering the use of flannelette sheets it is important to remember not to overdress baby, which may lead to overheating.
https://rednose.org.au/article/bedding-amount-recommended-for-safe-sleep
https://rednose.org.au/article/how-much-bedding-does-baby-need
Ensure there are no small “balls” of fabric that baby might be at risk of inhaling.