I always thought that Rilian was an only child. Maybe if he did have siblings, CS Lewis would have given some kind of hint. Then of course, he didn’t. 

So I always thought that Rilian was an only child, given the whole conflict in SC about Caspian about to die without an heir. 

At the end ofThe Voyage of The Dawn Treader, Lewis has this to say about Caspian and Ramandu the Star’s Daughter:

Caspian married Ramandu’s daughter and they all reached Narnia in the end, and she became a great queen and the mother and grandmother of great kings.

 

Yet, none of Rilian’s brothers are mentioned In The Silver Chair. 

That is an interesting observation.

I guess as with most things of this nature, the correct answer is that CS Lewis wrote one thing in VDT, and then either forgot about it or just ignored it for SC because it didn't fit with the story he was trying to tell.

Essentially though yes, in order to for the plot of Silver Chair to work, it really needs to be the case that Rilian is the only heir to the throne. Otherwise it opens up a whole other plot dynamic which would get in the way of everything else.

Maybe it's just the case that whilst she only had one child, Rilian had multiple children, therefore the plural "kings" only applies to the "grandmother" part.

It's possible Caspian and Ramandu's Daughter had children who died but I always interpreted things as Icarus suggests:

Posted by: @icarus

Maybe it's just he case that whilst she only had one child, Rilian had multiple children, therefore the plural "kings" only applies to the "grandmother" part.

Actually, I'd go further than that and say what the sentence is really supposed to mean is that she was the mother of Rilian, the grandmother of Rilian's son, the great grandmother of Rilian's grandson, etc. (It's also possible some of them were daughters/queens.) But just writing, "she became a great queen and the mother and grandmother of great kings." was simpler.

Plus, Tirian mentions in LB that he’s seventh in descent from Rilian and has been dead for over 200 years. “Grandmother” of great kings would consist of a lineage- Rilian, then eventually Erlian and Tirian. There would have obviously been kings between Rilian and Erlian. 

If Rilian is an only child, it's sad that after all the promise of the ending of VDT that Rilian and Stardaughter had so much trouble conceiving, considering Aslan mandated the match. There's an unsaid story in there somewhere.

Going by everything we know from the books, there's no way that Rilian could have been anything other than an only child.

The whole premise of the quest in SC is that the aged King Caspian is grieving, and has sailed east again after a rumour that Aslan may be out there, because he has no son to succeed him after his death. If Caspian and Ramandu's daughter had had children after Rilian, their next oldest son (or daughter if there were no other sons*) would automatically become heir to the throne. We see this in HHB, where Corin is heir to the throne of Archenland, until his long-lost 20-minutes-older twin brother Cor reappears — at which point Cor instantly becomes heir again.

So if Caspian had had other children after Rilian, no doubt he would grieve the disappearance of his eldest son, but he wouldn't be without an heir to the throne. Thus there would be no great incentive for anyone to go searching for Rilian in the years that followed — as we're told many have done and none have returned, leading Caspian to forbid anyone else doing this — let alone any incentive for the frail and elderly Caspian in his last days to go seeking for Aslan's advice on the matter. Rilian's disappearance would remain a sad mystery, but it wouldn't threaten to end the continuity of the Narnian monarchy in the way it clearly does in the book.

(Of course, if Rilian did have a younger brother or sister, the Green Witch could have returned and produced Rilian (under her control) as the rightful heir after his younger sibling had been crowned, throwing Narnia into a succession crisis. But we're given absolutely no indication that that was any part of the plot.)

On top of that, again from what we know of the books — and the way hereditary monarchies work — there's no way Lewis's remark about "the mother and grandmother of great kings" could mean that Ramandu's daughter was literally the mother of more than one king. That could only have happened if she'd had more than one son, and her eldest son (Rilian) had become king, but then died without having any children of his own — in which case the next oldest brother would become king, and therefore their mother would be the mother of more than one king.** Otherwise, even if she'd had other children, they would have remained princes and/or princesses at most, as long as Rilian fathered an heir in turn. Which he must have done, as Tirian is seventh in line from Rilian and therefore must be his direct descendant.

So "the mother and grandmother of great kings" can only logically be read as meaning Ramandu's daughter is the mother of one great king (Rilian) and the grandmother of another (Rilian's son, name unknown). That makes sense grammatically and it's the only reading that makes sense in the context of the stories themselves. No mystery and no controversy about it whatsoever.

I do agree it's curious that Caspian and his wife only ever had one child, but then again, it's what serves the purpose of the plot. Genetic incompatibility between stars and humans is an interesting idea, but I doubt it's something Lewis would have gone into in a children's book!  

* Assuming the Narnian monarchy follows the same male-heir-favouring rules that the British monarchy did until very recently.

** Incidentally, for anyone who enjoys history, the last time this happened in Britain in real life was with Queen Mary, wife of King George V. He died in 1936 and their eldest son became King Edward VIII, but he abdicated within less than a year — giving up the right for any future children of his to have a claim to the throne (though it turned out he never had children) — and his next-oldest younger brother became King George VI (father of the late Queen Elizabeth II). So Queen Mary was in fact the mother of two kings! But that hasn't happened ever since and isn't likely to in the near future, as in each generation of the Royal Family since then, the next one in line to the throne has already had children in turn. The succession normally goes straight down the family line from parent to child, and only passes "sideways" to a sibling — or to a nephew or niece — if there is no legitimate heir in that direct line. This happened even more interestingly in the early 1800s with the succession of George IV, William IV (his brother) and then Queen Victoria (their niece, daughter of their deceased brother), but that's getting way off topic!!  

Plus, in the books, there's no indications if any of the other royals had siblings. The closest we come to are-

Jadis and her sister

Frank and Helen's multiple children

The Pevensies (of course)

Cor and Corin

Caspian IX and Miraz

Caspian X was an only child, as well as Rilian. If Rilian had any siblings, there would be no quest for Eustace and Jill to go find the lost prince, and there would be no story!