As Alison mentions, would be good to see what info you have William born 1836 to see whether these posts relating to the US can be confirmed.
I saw elsewhere mention of youngest Agnes b. 1839 having married in 1863 (17 July 1863 Tradeston, Glasgow) an Archibald Walker. Trying to find them in 1871 census brought up this entry:
Archibald Walker 32 iron turner, b. Rothesay, Bute
Agnes Walker 30 b. Glasgow
William Walker 3
Archibald Walker 8 Months
William Mclean 32 boarder Card Guider b. Glasgow
The marriage cert should include full details on her parents to let you confirm and whether they were still alive at the time of the marriage.
Could this 1861 entry be Agnes and William, all born Glasgow. Not sure who James may be, born c. 1840?
William Mclean 24, iron turner
James Mclean 21 brother, Confectioner
Agnes Mclean 22 sister, cotton winder
Jessie Stevenson 20 lodger
Address: 100 South Wellington St, Glasgow Govan