I'm confused. If you are looking for a baptism of Christian Chalmers, and you don't know who her parents were, how do you know that the children of Robert Chalmers whose death certificates you have found were her siblings? Because if they were her siblings the death certificates would have told you who her parents were. From these death certificates, what was the name of the wife of Robert Chalmers, tollkeeper?
I think there may be two Robert Chalmers here, one of whom was a tollkeeper and the other a farm servant who had died before 1839.
Timeline
About 1800 Birth of Christian Chalmers or Finnie in Dalmeny, West Lothian
Before 1939 Death of Robert Chalmers
1839 William Finnie and Christian Chalmers marry. Marriage is recorded in both Edinburgh and Uphall
1841 William and Christian Chalmers or Finnie, 35 (so born 1801/6), born West Lothian in census in Uphall
1841 Christian Finnie, 42 (so born 1798/9), born Midlothian, in census in Dalkeith with (son?) Peter and (daughter?) Jessie. Therefore these are two totally different Christian Finnies
1851 William and Christina Chalmers or Finnie, 50 (so born 1800/1), born Dalmeny, in census in Uphall
1851 Christian Chalmers, 43 (so born 1797/8), in census in Rose Street, Edinburgh with husband David Chalmers. Therefore these are two different Christian Chalmers
1851 Christian Chalmers (or Finnie?) dies in Uphall
1853 William Finnie marries Christian Mackenzie