Assuming this is the man who served in the Pandora, it seems this Richard Mackey (baptised in Stronsay 1765) only had one sibling: Margaret. If that is the case, then a brick-wall of some description has appeared here. Unless his father Peter remarried and had sons (‘half brothers’) with another woman. Any other suggestions anyone?
Thanks to Brisbane-based family history researcher Lynda Hodgkinson for this detail; she sent it to me quite a while ago.