There is a will for a William Sinclair of Kirk Marown Parish , drawn up 22nd March 1827. He has no connection that I can determine with Henry Sinclair, but it contains a wealth of genealogical detail which may be of interest to other readers. You can view the original on line on LDS microfilm 0106427 starting image 76.
William was a farmer on the Estate of Craigbeg, quarterland of Ballaglioney, which he purchased from William Gellin.
Brother Alexander, a mason, of Newton Stewart Wigtonshire, married Margaret Martin. They had a son James, a tanner
Sister Jane of Glenluce, Wigtonshire married Thomas Dalrymple
Sister Isabel of Stranraer married William Douglas. They had a son James who in turn had a daughter Agnes
Sister Helen of Newton Stewart married William McIlwraith. They had a daughter Margaret.
Nephew John Sinclair (no spouse given) was a merchant in Newcastle upon Tyne
Also mentioned is Anne Bell spouse of George Maxwell, Prior's Linn , Canoby, Dumfries. Son George.