The bible confirms that Margaret Sloan married John Nicol pre 1861 but the Margaret Sloan born in Stair in 1833 was still living at home in 1861 as below from Ambly's post
"CENSUS 1861 At: 26 Burnside Street, St George Glasgow, Lanarkshire
Ref: Parish 644/7, ED 51, Page 3
Head: James SLOAN 58, Joiner, b Stair, Ayr (abt 1803)
Wife: Mrs James 57, b Stair, Ayr (abt 1804)
Dau: Margaret SLOAN 28, b Stair, Ayr (abt 1833)
Dau: Ann SLOAN 25, b Stair, Ayr (abt 1836)
Son: John SLOAN 21, Teacher Of Mathematical?, b Stair, Ayr (abt 1840)
Son: William W SLOAN 18, Student, b Glasgow (abt 1843) "
So the Margaret Sloan that John Nicol married pre 1861 would not have been the Margaret born 1833 in Stair so she must have been born elsewhere.
Using Jane's birth you gave from the bible in 1865 I think this is the right family in 1871 living at Main Street Grindlays House, Kilsyth after John's death with Margaret listed as born about 1835 in Kennap, Renfrewshire
Margaret Nicol 36 Housekeeper Hour b Kennap, Renfrewshire
William Nicol 15 b Barrhead
John Nicol 13 b Kilsyth
Thomas Nicol 12 b Kilsyth
Margaret Nicol 9 b Kilsyth
Elizabeth Nicol 8 b Kilsyth
Jane Nicol 5 b Kilsyth
I think by 1881 the widowed Margaret Nicol had remarried under her maiden name and was living at 55 Tylefield St, Glasgow with two of her children including Jane and two new ones from her marriage to Daniel. Margerat is listed as born Linwood, Renfrewshire
Daniel Trealower 40 Fireman b Cruwel, England
Margaret Trealower 47 b Linwood, Renfrewshire
Elizabeth Nicoll 17 b Kilsyth, Stirlingshire
Jane Nicoll 15 b Kilsyth, Stirlingshire
Bennett Trealower 7 b Kilsyth, Stirlingshire
Daniel Trealower 5 b Glasgow
I had free credits for Scotlands People from this offer
http://www.rootschat.com/forum/index.php?topic=745416.0;topicseen so I checked the marriage for Margaret and Daniel Trealower and attach an extract They married on 22 Aug 1871 at Kilsyth and Margaret lists herself as a widow with parents both deceased as Thomas Sloan (pedlar) and Martha Sloan maiden name Douglas
Looking for Margaret Sloan in 1851 premarriage to John Nicol with parents Thomas and Martha this could be her living at Main Street Reid's Land, Linwood,Renfrewshire. (The 1841 census for the same family lists Margaret as born in Scotland)
Thomas Sloan 60 Former Ag Worker b Ireland
Martha Sloan 56 b Ireland
Margaret Sloan 20 b Ireland
Thomas Sloan 16 b Linwood, Renfrewshire
The 1861 census for parents Thomas Sloan and Martha in Kilsyth lists them both as born in Belfast,Ireland and Thomas as a Pedlar
I do hope this isn't too confusing

Kay