Even in this sentence when there is no other 3rd person subject, you use “siajn”? I would have thought it was “iliajn”.
Yes. If you said "iliajn" it would mean that they visit someone elses grandparents.
The 3rd person subject is 'la nepinoj'
Wait. Sia can be used for plural 3rd person?
So "siajn" is for our own grandparents in this case and "iliajn" is for someone else? Does it right?
No, siajn is referring to the subject of the sentence. Iliajn would be someone else's parents.
to remember, the S in sia is like the s in subject.
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.
It is not ŝiajn but siajn. There is no ambiguity. It is a universal reflexive pronoun that always refers back to the subject.
Can "siajn" be singular? Is there a word to refer to one person of an unspecified gender like "them" in English?
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"
Why is it Siajn and not ŝiajn?
Because they are visiting their own grandparents.
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.