To date no one has been able to prove a marriage of Henri Cabanis and a Harrison. His first wife arrived with him 23 Jul 1700 and name Mary in application for land (after they were here 7 yrs as required). The application showed now wife Magdalene and his will ca 1720 named wife Mary. This Mary and Magdalene could be the same - guess work.
As to mother of Matthew and George, Allen Cabaniss in his book, "Cabaniss Through 4 Generations" name Mary (maiden name unknown) as their mother, but in a recent conservation with Alloa Caviness Anderson, the most knowledgeable Cabaniss researcher, and the most dependable, she stated that she was of the opinion that Magdelene had to be the mother of Matthew and George. She was using the date (1707 application for land) naming Magdelene and the approximate date of birth of Matthew, date established from land records, etc.
After carefully reviewing all the official documents I have, I tend to agree with Alloa, but neither have been proven!