Welcome to our group, John.

Do you know where Catherine was born? Or where they lived prior to arriving on PEI? Place of marriage?

You use the spelling Duglas, Douglas and Douglass. Does this reflect their names on records?

I have a note of a Margaret Louden Aitken born 24 Dec 1808 in Lower Montague, PEI, Canada, who died 3rd May 1852. She married William Alley, son of Thomas Alley and Catherine Douglas. They had 12 children.

Margaret was the daughter of George Aitken and Hannah Beairsto, who married 1st Aug 1798 in/near Queens Co., St. Johns Island, PEI.

There is a dubious reference to Douglas Castle, in Scotland.

Yours aye,

William

I initially thought that Carlowrock might be a mis-spelling of Carlingwark, but it appears that there was, once, a place called Carlowrock in Nithsdale.  However, I can find no trace of it now. I do have a note of an family called Lowden/Aitken resident there in the 18th century.

George Aitken, born  1779 and died 1857, son of John Aitken and Margaret Aitken (born Lowden).George Aitken, born  1779 and died 1857, son of John Aitken and Margaret Aitken (born Lowden).John was born in 1729, in Greenhill, Auchenhay, Kirkcudbrightshire Margaret was born in 1739, in Carlowrock, Nithsdale, George had 5 siblings including Margaret Aitken and Agnes AitkenGeorge married Hannah Aitken (born Beairsto) in 1798, at age 19 Hannah was born on October 10 1777, in West River, Prince Edward Island, Canada.They had 12 children including Margaret Louden Alley (born Aitken) and Anne Matilda Aitken
Passegers on the Lovely Nelly:John Aitken 50 Labourer Carlowrock CountrymanMargaret Lowden, wife 36James, son 17Goddion, son 7Margaret, daughter 4Agnews, daughter 2Reason for emigration, "To provide for his family a better livelihood."
There is a good account here: https://goo.gl/Evq3e2