There is a John son of Wm Steen (a manx form of stepham) + Issable clucas bapt Marown Mar 17760322 and mentioned in his mo's will 1791 - [note duplicated #]d 19 dec 1790;dau elinor;ch wm,john,esther,elinor + margt jt exors;son thomas off island- he was the youngest son + wouldn't inherit any land - however I can't find a will for husb wm - however the marriage contract for Wm = Issable is extremely interesting - dated 20? sep 1760;[day lost on fold]Thomas Stevenson(Bochin) + wife Ellinor als Moore obo William Stevenson illegitimate gson of sd Tho + illeg nephew to sd Ellinor; Thomas Clucas(Garth, marown) obo dau Isable; to marry within a month;Stevensons to give half estate of Bochin (that half Ellinor would be entitled to) ;young couple to pay £20;Clucas to give £25;witt Philip caine x, Robert Stevenson x, Thos fargher;

Thomas Stevenson had bought estate of Bitchin(various spellings) as explained in a later deed - dated 19 Nov 1768;recites Thomas Stevenson(Botchin) sometime ago purchased half estate Botchin from Wm Cubon + my now wife Elinor als Moore with my consent settled her half of estate on nephew William Stevenson + wife Isable in m/c[SSS May 1774 74] but reserving my half of estate. Now I give to son Robert (wholives in Marown on another part of estate belonging to Anne Cubon + her son) - baron rent 6s 8d + customs; Robt paying £17 10s to my exors to discharge money I borrowed to purchase sd property + another 33 to my legitimate gson Wm Stevenson wholives with me;witt Thos Fargher, Robert Kelly

I have no proof that this is your fam but not many Stevensons (any spelling!) in Marown
Marown registersare quite defficient - I transcribed Bishops transcripts post 1733 - see may site www.manxnotebook.com (tab line above)