Dear Monica,
Perhaps Alexander Laurie is a sibling of Catherine and Mary Laurie. This is exciting, as we have not been able to find any evidence of other children.
Peter Laurie and Mary McNiel Laurie, have been our toughest nut to crack. Can't find their births. We do have a copy of the OPR showing their marriage in Tynron, but, of course the OPR does not give any extra info, such as their age, parents' names, etc.
Catherine Laurie married Robert Grierson in 1855. The 1861 Census shows Mary Laurie living with them in Glasgow.
We appreciate your help. Thank you, Bonnie Lassie on Florida's Spacecoast.