I would
expect Kilbarchan to be Isabella's PoB although I'm not sure what
"Kilbarrick, Argylleshire" says on the original doc. but I couldn't find a place of that name?
Her age is consistent on each census.
https://www.freecen.org.uk/cgi/search.plPiece SCT1841/568
E D 10
Parish Kilbarchan (Renfrewshire )
Folio 13
Page 17
Address Ewing Street
Jackson (all born Renfrew)
Robert 67
Isabella 40
Janet 20
Agness 18
Jean 13
Robert 7
Not all children were baptised & not all records have survived.
Sometimes the Scottish naming pattern can help to establish possible parentage although in this case it’s not so easy to try & fathom as there are a lack of males.
You have established Roberts’ parents to be Robert & Janet i.e. my guess would be Isabella’s mother was possibly Agnes but as there seems to have been no other male children it’s anyone’s guess as to a name for her father?
Baptisms I could find except Jean;
Janet 1818
Agnes 1821
Margaret 1824
Jean c1826 (from 1841 census)
Isabel 1830
Robert 1833
Are there any clues on the above baptisms (sponsors) which give pointers?
Annie