Sons of Peter Robinson (Isaac², Rev. John¹) and wives:
By first wife, Mary Manter:

Peter Robinson Jr., born December 07, 1694 in Tisbury, Dukes (Martha's Vineyard), MA; died March 22, 1785 in Scotland Parish, Windham, Windham, CT. He married Ruth Fuller June 20, 1725 in Mansfield, Windham-Tolland, CT. (Children would have been born 1725-1745, possible grandfather.) [Unlikely]

Peter Robinson, Jr., is thought to have had 13 children born between 1726 and 1748. Most were born and probably lived in Preston, which is now part of New London Co., CT. Seven of these children were sons and several were old enough to be our Isaac's father. None of his sons were named Isaac. Only two seem to be likely prospects to have been the father of our Isaac:

Samuel Robinson, born July 06, 1726 in Windham, CT. He married Mary Kimball, probably about 1748. She was born 1729. (Children could have been born 1749-1769) [Eliminated]

• He had eleven children born between 1749 and 1768, no sons were named Isaac.

Peter Robinson, born May 19, 1730 in Windham, CT; died July 17, 1778 in Windham, CT. He married Abigail Palmer November 13, 1755. She was born March 26, 1733, and died September 25, 1774, and he married Sarah West, May 14, 1775. (Children could have been born 1748-1785) [Eliminated]

• Six children are known from his first marriage, there was no Isaac. No children are known from his second marriage, but it was too late.

There were other sons, who could have been the father of a child born about 1759, but they married and lived in Preston, New London Co., CT. [Unlikely]

Israel Robinson, born March 16, 1695/96 in Tisbury, Dukes (Martha's Vineyard), MA; died January 12, 1775 in Windham, Windham, CT, married(?). (Children would have been born 1715-1735, possible grandfather.) [Unlikely]

His three sons are all possible fathers:

Elisha Robinson, born February 05, 1724/25 in Windham, Windham, CT. He married Thankful Abt. 1750 in CT. (Children could have been born 1750-1770; no known children.) [Unlikely]

Daniel Robinson, born January 18, 1732/33 in Windham, Windham, CT. (Children could have been born 1752-1772; no known wife or children.) [Unlikely]

Eliezer Robinson, born July 08, 1734 in Windham, Windham, CT. (Children could have been born 1754-1774; no known wife or children.) [Unlikely]

Thomas Robinson, born October 23, 1698 in Tisbury, Dukes (Martha's Vineyard), MA; died March 28, 1783 in Windham, Windham, CT. He married (1) Annah 1724 in Windham, CT. He married (2) Abigail Dimick February 08, 1770 in Windham, CT. (Children would have been born 1724-1744, possible grandfather.) [Unlikely]

Two of his three sons are possible fathers:

Asa Robinson, born October 17, 1726 in Scotland Society, Windham Co., CT, died Unknown. He married Mary French. (Children could have been born 1744-1780.) [Eliminated]

• Eight children were born between 1749 and 1772, no Isaac.

Levi Robinson, born March 09, 1738/39 in Scotland Society, Windham, CT, died Unknown. (Children could have been born 1757-1790; no known wife or children.) [Unlikely]

By second wife, Experience Manter:

Simeon Robinson, born March 22, 1701/02 in Tisbury, Dukes (Martha's Vineyard), MA; died June 1792 in Windham, Windham, CT. He married Jerusha Kingsley February 02, 1742/43 in CT. (Children would have been born 1742-1762, possible father.) [Eliminated]

• Nine children are known from this marriage, and there is no Isaac.

Benjamin Robinson, born February 23, 1702/03 in Tisbury, Dukes (Martha's Vineyard), MA; died April 12, 1772 in Stafford Springs, Windham-Tolland, CT. He married (1) Jerusha Bingham March 04, 1728/29 in Windham, Windham, CT. He married (2) Katharine Durand abt. 1749. (Children would have been born 1728-1759, possible grandfather/father.) [Unlikely]

Two of his sons by the first wife are possible fathers:

Elijah Robinson, born October 01, 1735, he married Lydia Scripture, born April 21, 1744 in Coventry, CT. (Children could have been born 1761-1784.) (Possible, but unlikely parents.) [Unlikely]

Eliphalet Robinson, born December 05, 1739, he married Mary Folit November 25, 1761, born about 1743. (Children could have been born 1761-1779.) [Unlikely]

He is a possible father by the second wife, but five children are known born between 1749 and 1759 and no Isaac. [Eliminated]

Joseph Robinson, born February 04, 1705/06 in Tisbury, Dukes (Martha's Vineyard), MA; died July 01, 1789 in Scotland Parish, Windham, Windham, CT. He married Mehitable Reed July 17, 1735 in Windham, Windham, CT. (Children were born 1734-1758, possible grandfather/father.) [Unlikely]

Twelve children are known from this marriage, no Isaac. Two of his sons are old enough to have had a child born by 1759.

Joseph Robinson, b. 1743, Prob. Windham, Windham, CT. (No known wife or children, very young.) [Unlikely]

Moses Robinson, b. 1744, Prob. Windham, Windham, CT. (No known wife or children, very young.) [Unlikely]

Isaac Robinson, (twin of Anna), born August 05, 1708 in Tisbury, Dukes (Martha's Vineyard), MA; died December 03, 1796 in Windham, Windham, CT. He married Deborah Hibbard December 15, 1737 in Windham, Windham, CT. (Children would have been born 1737-1757, possible grandfather/father.) [Unlikely]

His sons are all unlikely fathers of our Isaac:

John Robinson, b. December 31, 1739, Windham, Windham, CT. He married Eunice Phelps February 02, 1764 in Coventry, Windham-Tolland Co., CT. (Children would be born 1764-1784.) [Unlikely]

Isaac Robinson, b. October 12, 1748, Windham, Windham, CT; d. Bef. 1752. [Eliminated]

Isaac Robinson, b. April 15, 1752, Windham, Windham, CT; d. Lebanon, Grafton, NH; m. (1) Joanna Coleman, October 26, 1775, South Coventry, Windham-Tolland, CT; b. Abt. 1755; m. (2) Emeline Weaver, Abt. 1780; b. Abt. 1760. (Too young.), (Note this is the Isaac who married Joanna Colemen in 1775.) [Eliminated]

Nathaniel Robinson, b. April 15, 1752, Windham, Windham, CT. (too young.) [Eliminated]

Note: Tolland County, was formed in 1785, until then, all of what is now Tolland Co. was part of Windham County.
The father of our Isaac would probably have been born 1700-1745, his mother 1719-1746.

CONCLUSION: Based on the above, I have found it very unlikely that any of the descendants of this Peter Robinson, for one reason or another, could have been our Isaac's father. So I went another route, see that at Possible Ancestors of Isaac Robinson, II