Okay, I think I've figured out some things.
I found that a Gawen Harris Nash married Mrs Sarah Luttman Jan 23, 1747.
Their daughter Jane was baptized Feb 26 1748.
So I am assuming that the Gawen Harris Nash whose will and codicil was dated 1787, was the son, from a previous marriage, of the Gawen Harris Nash who married Sarah Luttman.
Jane, his step sister, would be considered his "sister, in law".
So now, Looking for marriage of Jane Nash to a Yeomans.