Are there any recommendations for the fabric used for cot sheets? Is it best to stick with cotton or are jersey or flannelette sheets ok as long as the other safe sleep recommendations are followed? Thank you

There is no specific fabric required for cot sheets as part of Red Nose Safe sleep recommendations.
Red Nose does recommend the use of light weight blankets tucked in firmly & light weight cotton or muslin for wraps/swaddles.
It is important that any bedding does not cover baby’s head/face &/or increase the risk of overheating.
https://rednose.org.au/article/how-much-bedding-does-baby-need