I have transcribed the presentments for 1817-1822 but no Isabella Sayle - not all women producing illegitimate children were presented especially in Douglas as the vicar + wardens generally did not concern themselves with it

I have no note of any Isabella Sayle producing a son Wm in period 1815-1825 - again my list may be incomplete

looking for Wm Sayle in 1841 manx census of the age suggested suggests only one in Peel but I have a good match for him.

I think you need to determine what you are confident about and then work back - not all family produced trees are correct or they may be confused as to date or place