I should probably read your other threads related to this family as this may already have been covered ... but ....
On Ancestry on the Assisted Immigrant Passenger lists have you seen the image after Christys? It is for a Bell Munro "a member of Donald McDonald's family". Her father is Donald Munroe, a farmer in Dornoch, and her mother is Janet McKenzie. Bell is from Rosskeen.
Obviously not a sister to Christie but maybe a cousin? This family might be worth looking for?
The way the forms are filled in father's first name and surname is given and mother's first name and maiden name is given. Christie's father is John Munroe and her mother is Margaret Munroe.
If correct I am therefore presuming that Munroe is Margaret's maiden surname as well as her married surname?
Looking on Scotland's People I can see two marriages of a John Munroe and a Margaret Munroe which may fit with a couple having children in the early 1800s. One in Rosskeen in 1815 and one in Alness in 1795. If you do a search on SP for children with surname Munro* and parents John and Margaret there seem a few results. I have not checked thoroughly but did not see an obvious Christie or variants with those parents, but finding other children might lead to finding Chris*.