john cook was not a loyalist. He fought against the british in the revolutionary war. One confirmed sibling is Thomas Cook. Thomas was "of Hanover, Massachusetts" when he married at Pembroke, Massachusetts in 1770, Hannah Lincoln, of Pembroke. Thomas was in Pembroke, Massachusetts in 1795 when he signed off on a deed with 2nd Wife Mercy Garnett. Mercy, her siblings and mother selling land owned by Mercy's father. Shortly after the sale, Thomas' daughther Hannah Cook and her Husband Laban Garnett (Gardner) moved to Dennysville ME, later named Pembroke, Maine. Her uncle, this John Cook already lived there, moving there in about 1790. Can find no other trace of brother Thomas Cook. I don't know if he also ventured to Maine, or stayed in Massachusetts. Two of his children, Peleg Cook and Lydia cook, married at Pembroke, in 1801 and 1803.
I suspect that James Cook of Hingham, who married Lucy Hall is also a brother. John Cook's revoluationary war papers indicate John cook's "parents and brother lived in Hingham". This would have been early 1780s. There is a James Cook, who married Lucy Hall, named one son James, the other John. James also served in the Revoluationary war, as did John and Thomas. Thomas is confirmed a brother of John, also as evidenced in John Cook's papers. Hannah Cook Garnett (Gardner) Thomas's daughter, who after 1795 joined her uncle in Pembroke, ME, submitted testimony that "John Cook dec. husband of Lydia Garnett was my uncle".
Other son, Peleg Cook also filed papers which indicated he is an heir to John Cook, who fought as a private in the revoluationary war, and John Cook was his grandfather, father of Thomas Cook, Peleg's father. Regrettably, now stuck at John Cook, father to Thomas and John, and most probably James. If you are researching this family, would be happy to compare notes/etc.