I cannot find an Isabella Tewnion on Scotland’s People born Banff about 1852
The nearest I can find for this unusual name is Helen Tewnion born Cullen 1852 Parents James Tewnion & Isabella Reid.
I could not find Isabella in 1861 but I 1871 Isabella Tewnon age 18 is with a James Tewnon age 13 at Marnoch, Banff.
In 1861 James Tewnon age 3 is in Marnoch, with Jessie age 25 and Alexander age 1.
You would need to see original census for details.
The 1881 census transcriptions are available on various sites by that time there were a number of TUENION/TEWNON in Marnoch, Banff, including Isabella age 27 born Boyndie, Banff and her son William age 1 (Handicapped) born Aberchirder, Banff.
I could not find a marriage for this Isabella.
Regards
Colin