Why? The ambiguity on this sxiajn / ilijn thing isn't an issue in any other language I know, including English.
Furthermore, 'sxiajn' could still be referring to a 3rd person singular (like Jack and Jill visiting Molly's parents) so the ambiguity remains nonetheless.
Yes, it would be "sian" or "sia." And yes, it is "gxi," which would be rude to call slmeone in engish "here it is," but is it fine in Espersnto-"Jun estas gxi"
Had it been ŝiajn, it would have been about "ŝia" (her) grandparents. The context would have to explain who this other female is that the sentence would be talking about in that case.