I have read it is not advised to use sheepskin in the cot. My mother in law has a cot at her house and has placed a sheepskin under the fitted sheet. Is it still a risk? Or because it is under the fitted sheet and not in direct contact, is that ok? Thankyou.

Red Nose recommends to sleep baby on a mattress that is firm, clean, well-fitting & flat (not elevated)
A soft mattress or sleeping surface can increase the risk of sudden unexpected infant death if baby rolls over onto the tummy.
Remove plastic packaging from the mattress and always make sure that the waterproof mattress protector is strong and a tight fit.
Never put soft bedding under the bottom sheet, such as a sheepskin, as this makes the sleeping surface too soft.
https://rednose.org.au/article/what-is-a-safe-mattress