Have you looked for them at Scotland's People
www.scotlandspeople.gov.uk (SP)?
According to the church records there James William, son of William Sievwright and Jane Machray, was baptised in Lumphanan on 5 January 1853. I took a look at that. The original document says, "
Sept 1st 1852. James William, natural son of William Sievwright, farmer, Mains of Auchlossie, and Jane Machray in Wester Kincraigie, was born; and was baptized on the 5th day of January 1853".
In the 1861 census he is probably the child listed as William, a boarder, aged 8, in a household in Aboyne.
Jane Machray, aged 21, is living with her widowed mother and her siblings at Kincraigie, Lumphanan, in 1851.
William Sivewright, unmarried, 29, farmer, born Drumoak, is in the 1851 census at Auchlossen, Lumphanan, with his widowed mother, three sisters and several servants including one who is possibly Jane's brother.
I have not yet found either William or Jane in the 1861 census.
You can view the originals of all these documents, and a lot more, on the SP web site at modest cost.
Hint: When searching on SP, use wildcards. S*v*r*t will pick up all the spelling variations of Sievewright.