In the 1600s and early 1700s, "cousin" was used to mean "niece" and "nephew," as well as the standard cousin definition that we know today. A lot of terms had entirely different meanings back then than they do today, so the key is to look at the term in context of the time it was used. Kind of like the way "son-in-law" back then meant the equivalent of today's "step-son," NOT today's definition of "son-in-law." Not understanding the context has led to a LOT of misunderstanding of family relationships from the 1600s and early 1700s. What we know today as "in-law" relationships back then were noted as regular relationships without using the "in law" term -- hence Abraham Field referring to Patrick Spence as his "brother" in his will when they were actually brothers-in-law through their wives.
I don't know anything more about Patrick and Dorcas' daughter Mary, other than her mention in the deed of horses from Dorcas' sister Mary (--) Bell Field Ironmonger Gallent to the Spence children. Quite the black widow was Dorcas' sister Mary!