I'd suggest that Wm s/o edward kissack + isable cowle bapt 2 feb 1742/3 might be worth checking out - Rex Kissack has nothing to say about both Wm and his sibling Edward - father edward died 1774 and seems to have left a will 1775 (not yet transcribed), Isabel d 1785 also leaving a will (also not yet transcribed) - I can't find a burial for edward but there is a will 1778 of an edward for whom no obvious burial can be found which may also be useful. I can't find a burial for wm but he is not mentioned in his sister Christian's will of 1780 though that may not indicate he was dead as she had eldest bro John as exor and mentions her sisters and mother (that will is summarised on manxnotebook)