This is a bewildering family and unfortunately back before the censuses and statutory registration it is hard to really know. The names of Marion's parents do make it look like they were following the Scottish naming pattern. In that case Roderick's parents should be Hector and Betsy.
From children's births on FamilySearch.org it looks like Alexander Stewart and Marrion McLean were having children in Kiltearn, Ross & Cromarty. There is a Helen Stewart christened on 8 Apr 1798 and an Abbey Stewart christened on 23 Feb 1800. If Roderick and Marion came from the same place and their families knew each other before they emigrated, then that looks more like this one:
Roderick Mackenzie, christened 15 Dec 1799 Kiltearn, Ross and Cromarty, parents Alexander Mackenzie and Margaret Fraser.
… which doesn't fit with the naming pattern, but does have a father Alexander!

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Sarah