My reasoning

for thinking that Nancy McClain was more likely John's sister than his daughter is because I think it would be really odd to leave a daughter one schilling, leaving another daughter a cow, having the estate divided among your sons and then making the husband of that one schilling daughter the administrator of the will.  Knowing that Joseph's wife was Nancy, and Nancy is not mentioned in the will, makes me think that Nancy McClain was not John's daughter.  And, remember, James the son had Joseph removed as administrator.  I think the sons wanted to sell the land and Joseph did not want to.  They had already sold everything except the land.