Jane McKinnon was Gregor's granddaughter, d/o Hugh and Jane (Forsyth) McKinnon.
JANE McKINNON b. ca. 1832-3, d. Pointe aux Car, Glenelg Parish, Northumberland Co., NB 21JUN1853 age 21 years, m. the Manse, Chatham, Chatham Parish, Northumberland Co., NB by the Rev. Wm. Stewart 8APR1852 ROBERT RUSSELL (b. ca. 1825; m. 2nd by the Rev. Wm. Stewart 21JAN1857 Margaret Stuart (b. ca. 1838), d/o Charles and Helen Stuart), s/o Francis and Ann Russell.
As noted above, Jane McKinnon b. ca. 1833 appears with John McDonald (b. ca. 1814) and his wife, Margaret (b. ca. 1823) in the 1851 census of Glenelg Parish, Northumberland Co., NB. The eldest son of John and Margaret is Francis McDonald (b. ca. 1842), suggesting they were married ca. 1840. As it turns out, Francis' forename suggests more than that.
Jane is given, in 1851, as a cousin to John and Margaret McDonald. An attempt to find a McKinnon-McDonald kinship was fruitless. Turning to the possibility that Jane and Margaret were cousins, a marriage worth noting is that of John McDonald at Point aux Car, by A. Goodfellow, Esq. 6JAN1840 to Margaret Russell, eldest d/o F. Russell, all of Glenelg Parish. The timing and the fact that Jane's cousins are the only John and Margaret McDonald in Glenelg Parish in 1851 suggest they are the same persons.
F. Russell, Margaret's father, lives very close in 1851 (only one page removed in the census returns), and is Francis Russell b. NB ca. 1790, with a wife, Ann (b. NB ca. 1793) and a son, Robert (b. ca. 1825). there is also a Francis R. Taylor b. ca, 1843, likely a grandson, and suggesting at least one older, married daughter. Robert Taylor would also be the same who m. Jane McKinnon. Thus, while the 1851 census seems to anticipate a bit, Jane and Margaret (Russell) McDonald will soon be related as sisters-in-law.
It should be noted that mentions of Hardwick Parish, where Hugh's mother died, are not inconsistent as Hardwick was set off from Glenelg Parish in 1851, just after the census. Thus a change of 'place' does not mean an actual movement of persons. Also, John McDonald, one of the first families listed in 1851, seems to have lived near a boundary of the Parish. Thus, when a newspaper notes Jane's marriage as:
"m. Manse, Chatham (North. Co). Thursday 8th inst., by Rev. Wm Stewart, Robert RUSSELL, Glenelg / Miss Jane MCKINNON, Hardwicke"
the mention of Hardwicke Parish is not only consistent, but supportive.
Francis Russell is mentioned in Doreen M. Arbuckle's, "The North West Miramichi", pgs. 211-215; and in Kathleen Willis Williston's, "Biographies and notes on early settlers" written by Mrs. Williston in 1947. In summary, he is given as b. Scotland ca. 1790, d. 1874, m. 4FEB1813 Ann Forsyth (b. ca. 1793), d/o Robert and Jane (Martin) Forsyth.
These circumstances strongly suggest that Jane McKinnon and Margaret (Russell) McDonald were first cousins; therefore Jane was the daughter of Margaret's maternal aunt (paternal aunt being unlikely), therefore Hugh McKinnon's wife, Jane, was Jane Forsyth, d/o Robert and Jane (Martin) Forsyth, as suspected. Also, that Jane McKinnon's husband, Robert Russell, is her maternal first cousin.
Francis Russell (81) lived in Glenelg Parish, Northumberland Co., NB in 1871. With him was his son, Robert (45), Robert's wife, Margaret (32) and Robert and Margaret's children. Next door was Angus Russell (50), and his wife, Jane (47). Angus and Jane had been in Newcastle Parish in 1851. With them had lived Alexander Taylor (ca. 1812), apparently a neighbour from Glenelg Parish. Their eldest child, Charles Russell, was b. ca. 1858 suggesting that if Robert and Jane had any children, none survived her.
By 1881, in Glenelg Parish, Robert Russell b. ca. 1825 and his wife, Margaret, b. ca. 1838. Children are: Charles (23), Francis (21), Angus (18), John (16; John A. Russell d. while bathing Berlin Falls, NH 4JUL1886), James (13), Arthur (11), Clifford (8), Harry (6), David (4) and Ellen (1). Robert is a farmer-fisherman and Charles and Francis are carpenters. All are b. NB, Scottish and Presbyterian.
I think Jane Forsyth had a brother, James Batey Forsyth, who m. Margaret Russell likely a d/o Francis. Another brother, Robert Forsyth, m. Hugh McKinnon's sister, Margaret McKinnon. So, as you suggest, there was likely an extended kinship between these families.